diff --git a/opencensus/trace/internal/running_span_store_impl.cc b/opencensus/trace/internal/running_span_store_impl.cc
index cdcc7f6e..4fbd3f37 100644
--- a/opencensus/trace/internal/running_span_store_impl.cc
+++ b/opencensus/trace/internal/running_span_store_impl.cc
@@ -34,28 +34,21 @@ namespace opencensus {
 namespace trace {
 namespace exporter {
 
-namespace {
-// Returns the memory address of the SpanImpl object, to be used as a key into
-// the map of spans.
-uintptr_t GetKey(const SpanImpl* span) {
-  return reinterpret_cast<uintptr_t>(span);
-}
-}  // namespace
-
 RunningSpanStoreImpl* RunningSpanStoreImpl::Get() {
   static RunningSpanStoreImpl* global_running_span_store =
       new RunningSpanStoreImpl;
   return global_running_span_store;
 }
 
-void RunningSpanStoreImpl::AddSpan(const std::shared_ptr<SpanImpl>& span) {
+void RunningSpanStoreImpl::AddSpan(const SpanId& id,
+                                   const std::shared_ptr<SpanImpl>& span) {
   absl::MutexLock l(&mu_);
-  spans_.insert({GetKey(span.get()), span});
+  spans_.insert({id, span});
 }
 
-bool RunningSpanStoreImpl::RemoveSpan(const std::shared_ptr<SpanImpl>& span) {
+bool RunningSpanStoreImpl::RemoveSpan(const SpanId& id) {
   absl::MutexLock l(&mu_);
-  auto iter = spans_.find(GetKey(span.get()));
+  auto iter = spans_.find(id);
   if (iter == spans_.end()) {
     return false;  // Not tracked.
   }
@@ -63,6 +56,18 @@ bool RunningSpanStoreImpl::RemoveSpan(const std::shared_ptr<SpanImpl>& span) {
   return true;
 }
 
+bool RunningSpanStoreImpl::FindSpan(const SpanId& id,
+                                    std::shared_ptr<SpanImpl>* span) {
+  absl::MutexLock l(&mu_);
+  auto iter = spans_.find(id);
+  if (iter != spans_.end()) {
+    *span = iter->second;
+    return true;
+  } else {
+    return false;
+  }
+}
+
 RunningSpanStore::Summary RunningSpanStoreImpl::GetSummary() const {
   RunningSpanStore::Summary summary;
   absl::MutexLock l(&mu_);
diff --git a/opencensus/trace/internal/running_span_store_impl.h b/opencensus/trace/internal/running_span_store_impl.h
index 4d8fe2e2..da8c7a7b 100644
--- a/opencensus/trace/internal/running_span_store_impl.h
+++ b/opencensus/trace/internal/running_span_store_impl.h
@@ -38,11 +38,16 @@ class RunningSpanStoreImpl {
   static RunningSpanStoreImpl* Get();
 
   // Adds a new running Span.
-  void AddSpan(const std::shared_ptr<SpanImpl>& span) LOCKS_EXCLUDED(mu_);
+  void AddSpan(const SpanId& id, const std::shared_ptr<SpanImpl>& span)
+      LOCKS_EXCLUDED(mu_);
+
+  // Finds a Span with id SpanId, returns false if not found.
+  bool FindSpan(const SpanId& id, std::shared_ptr<SpanImpl>* span)
+      LOCKS_EXCLUDED(mu_);
 
   // Removes a Span that's no longer running. Returns true on success, false if
   // that Span was not being tracked.
-  bool RemoveSpan(const std::shared_ptr<SpanImpl>& span) LOCKS_EXCLUDED(mu_);
+  bool RemoveSpan(const SpanId& id) LOCKS_EXCLUDED(mu_);
 
   // Returns a summary of the data available in the RunningSpanStore.
   RunningSpanStore::Summary GetSummary() const LOCKS_EXCLUDED(mu_);
@@ -61,8 +66,13 @@ class RunningSpanStoreImpl {
 
   mutable absl::Mutex mu_;
 
-  // The key is the memory address of the underlying SpanImpl object.
-  std::unordered_map<uintptr_t, std::shared_ptr<SpanImpl>> spans_
+
+  // Necessary for using SpanId as the key in the map.
+  struct SpanIdHash {
+    std::size_t operator()(SpanId const& s) const noexcept { return s.hash(); }
+  };
+
+  std::unordered_map<SpanId, std::shared_ptr<SpanImpl>, SpanIdHash> spans_
       GUARDED_BY(mu_);
 };
 
diff --git a/opencensus/trace/internal/span.cc b/opencensus/trace/internal/span.cc
index 36e648d6..fd1f8e01 100644
--- a/opencensus/trace/internal/span.cc
+++ b/opencensus/trace/internal/span.cc
@@ -69,13 +69,22 @@ class SpanGenerator {
     SpanId span_id = GenerateRandomSpanId();
     TraceId trace_id;
     SpanId parent_span_id;
-    TraceOptions trace_options;
+    TraceOptions trace_options = options.trace_options;
     if (parent_ctx == nullptr) {
       trace_id = GenerateRandomTraceId();
     } else {
       trace_id = parent_ctx->trace_id();
       parent_span_id = parent_ctx->span_id();
       trace_options = parent_ctx->trace_options();
+      if (trace_options.HasStrictSpans() && trace_options.IsSampled() && !has_remote_parent ) {
+        std::shared_ptr<SpanImpl> span;
+        if (exporter::RunningSpanStoreImpl::Get()->FindSpan(parent_span_id,
+                                                            &span)) {
+          span->IncrementChildCount();
+        } else {
+            assert(false);
+        }
+      }
     }
     if (!trace_options.IsSampled()) {
       bool should_sample = false;
@@ -135,10 +144,15 @@ Span Span::StartSpanWithRemoteParent(absl::string_view name,
 Span::Span(const SpanContext& context, SpanImpl* impl)
     : context_(context), span_impl_(impl) {
   if (IsRecording()) {
-    exporter::RunningSpanStoreImpl::Get()->AddSpan(span_impl_);
+    exporter::RunningSpanStoreImpl::Get()->AddSpan(context.span_id(),
+                                                   span_impl_);
   }
 }
 
+void Span::IncrementChildCount() const {
+  span_impl_->IncrementChildCount();
+}
+
 void Span::AddAttribute(absl::string_view key,
                         AttributeValueRef attribute) const {
   if (IsRecording()) {
@@ -204,13 +218,38 @@ void Span::SetStatus(StatusCode canonical_code,
 
 void Span::End() const {
   if (IsRecording()) {
-    if (!span_impl_->End()) {
-      // The Span already ended, ignore this call.
-      return;
-    }
-    exporter::RunningSpanStoreImpl::Get()->RemoveSpan(span_impl_);
-    exporter::LocalSpanStoreImpl::Get()->AddSpan(span_impl_);
-    exporter::SpanExporterImpl::Get()->AddSpan(span_impl_);
+     EndSpanById(context_.span_id());
+  }
+}
+
+void Span::EndSpanById(const SpanId& id) {
+  std::shared_ptr<SpanImpl> span;
+  auto found = exporter::RunningSpanStoreImpl::Get()->FindSpan(id, &span);
+  assert(found && "Cannot call end span on an invalid ID");
+  if (!found) {
+     return;
+  }
+
+  if(!span->End()) {
+     return;
+  }
+  exporter::RunningSpanStoreImpl::Get()->RemoveSpan(id);
+  exporter::LocalSpanStoreImpl::Get()->AddSpan(span);
+  exporter::SpanExporterImpl::Get()->AddSpan(span);
+
+  if (span->context().trace_options().HasStrictSpans() &&
+      (!span->remote_parent() &&
+       span->parent_span_id().IsValid())) {
+     std::shared_ptr<SpanImpl> parent_span;
+
+     found = exporter::RunningSpanStoreImpl::Get()->FindSpan(
+       span->parent_span_id(), &parent_span);
+     assert(found && "Parent Span Id Not Found in Strict Spans Mode");
+
+     if (!found) {
+        return;
+     }
+     parent_span->DecrementChildCount();
   }
 }
 
diff --git a/opencensus/trace/internal/span_id.cc b/opencensus/trace/internal/span_id.cc
index 20600e5f..ac36a874 100644
--- a/opencensus/trace/internal/span_id.cc
+++ b/opencensus/trace/internal/span_id.cc
@@ -15,6 +15,7 @@
 #include "opencensus/trace/span_id.h"
 
 #include <cstring>
+#include <functional>
 #include <string>
 
 #include "absl/strings/escaping.h"
@@ -42,6 +43,15 @@ bool SpanId::IsValid() const {
   return tmp != 0;
 }
 
+std::size_t SpanId::hash() const {
+  // SpanIds are currently 64 bits. this hash function is only ever used
+  // internally within a given process, so this can be easily changed if we
+  // ever increase the size.
+  std::hash<uint64_t> hash_fn;
+  uint64_t val = *(reinterpret_cast<const uint64_t *>(rep_));
+  return hash_fn(val);
+}
+
 void SpanId::CopyTo(uint8_t *buf) const { memcpy(buf, rep_, kSize); }
 
 }  // namespace trace
diff --git a/opencensus/trace/internal/span_impl.cc b/opencensus/trace/internal/span_impl.cc
index 66ec913d..400aef78 100644
--- a/opencensus/trace/internal/span_impl.cc
+++ b/opencensus/trace/internal/span_impl.cc
@@ -73,8 +73,6 @@ std::unordered_map<std::string, exporter::AttributeValue> CopyAttributes(
 }
 }  // namespace
 
-// SpanImpl::SpanImpl() : has_ended_(false), remote_parent_(false) {}
-
 SpanImpl::SpanImpl(const SpanContext& context, const TraceParams& trace_params,
                    absl::string_view name, const SpanId& parent_span_id,
                    bool remote_parent)
@@ -87,7 +85,9 @@ SpanImpl::SpanImpl(const SpanContext& context, const TraceParams& trace_params,
       links_(trace_params.max_links),
       attributes_(trace_params.max_attributes),
       has_ended_(false),
-      remote_parent_(remote_parent) {}
+      remote_parent_(remote_parent),
+      active_child_count_(0),
+      end_requested_(false) {}
 
 void SpanImpl::AddAttributes(AttributesRef attributes) {
   absl::MutexLock l(&mu_);
@@ -139,6 +139,10 @@ void SpanImpl::SetStatus(exporter::Status&& status) {
 
 bool SpanImpl::End() {
   absl::MutexLock l(&mu_);
+  if (context_.trace_options().HasStrictSpans() && active_child_count_ > 0) {
+     end_requested_ = true;
+     return false;
+  }
   if (has_ended_) {
     assert(false && "Invalid attempt to End() the same Span more than once.");
     // In non-debug builds, just ignore the second End().
@@ -154,6 +158,25 @@ bool SpanImpl::HasEnded() const {
   return has_ended_;
 }
 
+void SpanImpl::IncrementChildCount() const {
+  absl::MutexLock l(&mu_);
+  active_child_count_++;
+}
+
+void SpanImpl::DecrementChildCount() {
+  bool shouldEnd = false;
+  {
+    absl::MutexLock l(&mu_);
+    assert(active_child_count_ > 0);
+    active_child_count_--;
+    shouldEnd = (active_child_count_ == 0 && end_requested_);
+  }
+
+  if (shouldEnd) {
+     Span::EndSpanById(context_.span_id());
+  }
+}
+
 exporter::SpanData SpanImpl::ToSpanData() const {
   absl::MutexLock l(&mu_);
   // Make a deep copy of attributes.
diff --git a/opencensus/trace/internal/span_impl.h b/opencensus/trace/internal/span_impl.h
index c1ff057a..bf540702 100644
--- a/opencensus/trace/internal/span_impl.h
+++ b/opencensus/trace/internal/span_impl.h
@@ -102,6 +102,15 @@ class SpanImpl final {
 
   SpanId parent_span_id() const { return parent_span_id_; }
 
+  bool remote_parent() const { return remote_parent_; }
+
+  // Increments the counter of open child spans below this span.
+  void IncrementChildCount() const;
+
+  // Decrements the open child counter AND if the counter becomes
+  // zero then End() is called on the span.
+  void DecrementChildCount();
+
  private:
   friend class ::opencensus::trace::exporter::RunningSpanStoreImpl;
   friend class ::opencensus::trace::exporter::LocalSpanStoreImpl;
@@ -138,6 +147,11 @@ class SpanImpl final {
   bool has_ended_ GUARDED_BY(mu_);
   // True if the parent Span is in a different process.
   const bool remote_parent_;
+  // Counts the number of open children linked to this span.
+  mutable uint32_t active_child_count_ GUARDED_BY(mu_);
+  // Keeps track if End() was called on this span while it had open child
+  // spans and is waiting on them.
+  mutable bool end_requested_ GUARDED_BY(mu_);
 };
 
 }  // namespace trace
diff --git a/opencensus/trace/internal/span_test.cc b/opencensus/trace/internal/span_test.cc
index 246d67b5..b2ecb392 100644
--- a/opencensus/trace/internal/span_test.cc
+++ b/opencensus/trace/internal/span_test.cc
@@ -282,6 +282,66 @@ TEST(SpanTest, FullSpanTest) {
   expect_message_event(3, 4, 5, data.message_events().events()[1].event());
 }
 
+TEST(SpanTest, StrictSpanTest) {
+  AlwaysSampler sampler;
+  TraceOptions t = TraceOptions().WithStrictSpans(true);
+  StartSpanOptions opts = {&sampler, {}, t};
+  auto span = ::opencensus::trace::Span::StartSpan("MyRootSpan", nullptr, opts);
+  auto child = Span::StartSpan("child", &span);
+  EXPECT_TRUE(span.context().IsValid());
+
+  // Calling end on the parent span with an open child should fail/not cause it to end
+  span.End();
+  const exporter::SpanData data = SpanTestPeer::ToSpanData(&span);
+  EXPECT_EQ("MyRootSpan", data.name());
+  EXPECT_EQ(false, data.has_ended());
+
+  // Now end the child and see that the parent span is ended as well
+  child.End();
+  const exporter::SpanData data2 = SpanTestPeer::ToSpanData(&span);
+  EXPECT_EQ("MyRootSpan", data2.name());
+  EXPECT_EQ(true, data2.has_ended());
+
+
+  // Now, try ending child first and verify that that doesn't end the parent span
+  auto span2 = ::opencensus::trace::Span::StartSpan("root2", nullptr, opts);
+  auto child2 = Span::StartSpan("child2", &span2);
+  EXPECT_TRUE(span2.context().IsValid());
+
+  // Now end the child and see that the parent span is ended as well
+  child2.End();
+  const exporter::SpanData root2_pre = SpanTestPeer::ToSpanData(&span2);
+  EXPECT_EQ("root2", root2_pre.name());
+  EXPECT_EQ(false, root2_pre.has_ended());
+
+
+  // Calling end on the parent span with an open child should fail/not cause it to end
+  span2.End();
+  const exporter::SpanData root2_post = SpanTestPeer::ToSpanData(&span2);
+  EXPECT_EQ("root2", root2_post.name());
+  EXPECT_EQ(true, root2_post.has_ended());
+
+
+  // Now try with 3 tiers
+  auto grandparent = ::opencensus::trace::Span::StartSpan("grandpa", nullptr, opts);
+  auto parent = ::opencensus::trace::Span::StartSpan("parent", &grandparent, opts);
+  auto kid = Span::StartSpan("kid", &parent);
+
+  // End both grandparent and parent, neither should have ended
+  grandparent.End();
+  parent.End();
+  const exporter::SpanData grandparent_open_data = SpanTestPeer::ToSpanData(&grandparent);
+  EXPECT_EQ(false, grandparent_open_data.has_ended());
+  const exporter::SpanData parent_open_data = SpanTestPeer::ToSpanData(&parent);
+  EXPECT_EQ(false, parent_open_data.has_ended());
+
+  kid.End();
+  const exporter::SpanData grandparent_closed_data = SpanTestPeer::ToSpanData(&grandparent);
+  EXPECT_EQ(true, grandparent_closed_data.has_ended());
+  const exporter::SpanData parent_closed_data = SpanTestPeer::ToSpanData(&parent);
+  EXPECT_EQ(true, parent_closed_data.has_ended());
+}
+
 TEST(SpanTest, ChildInheritsTraceOption) {
   constexpr uint8_t trace_id[] = {1, 2,  3,  4,  5,  6,  7,  8,
                                   9, 10, 11, 12, 13, 14, 15, 16};
diff --git a/opencensus/trace/internal/trace_options.cc b/opencensus/trace/internal/trace_options.cc
index 3d786bb8..59b46534 100644
--- a/opencensus/trace/internal/trace_options.cc
+++ b/opencensus/trace/internal/trace_options.cc
@@ -25,12 +25,15 @@ namespace trace {
 namespace {
 // One bit per option.
 constexpr uint8_t kIsSampled = 1;
+constexpr uint8_t kStrictSpans = 2;
 }  // namespace
 
 TraceOptions::TraceOptions(const uint8_t* buf) { memcpy(rep_, buf, kSize); }
 
 bool TraceOptions::IsSampled() const { return rep_[0] & kIsSampled; }
 
+bool TraceOptions::HasStrictSpans() const { return rep_[0] & kStrictSpans; }
+
 bool TraceOptions::operator==(const TraceOptions& that) const {
   return memcmp(rep_, that.rep_, kSize) == 0;
 }
@@ -49,5 +52,12 @@ TraceOptions TraceOptions::WithSampling(bool is_sampled) const {
   return TraceOptions(buf);
 }
 
+TraceOptions TraceOptions::WithStrictSpans(bool strict_spans) const {
+  uint8_t buf[kSize];
+  CopyTo(buf);
+  buf[0] = (buf[0] & ~kStrictSpans) | (strict_spans ? kStrictSpans : 0);
+  return TraceOptions(buf);
+}
+
 }  // namespace trace
 }  // namespace opencensus
diff --git a/opencensus/trace/internal/trace_options_test.cc b/opencensus/trace/internal/trace_options_test.cc
index fc0a2ab1..1bd19c6a 100644
--- a/opencensus/trace/internal/trace_options_test.cc
+++ b/opencensus/trace/internal/trace_options_test.cc
@@ -32,6 +32,10 @@ TEST(TraceOptionsTest, SetSampled) {
   opts = opts.WithSampling(false);
   EXPECT_EQ("00", opts.ToHex());
   EXPECT_FALSE(opts.IsSampled());
+  EXPECT_FALSE(opts.HasStrictSpans());
+  opts = opts.WithStrictSpans(true);
+  EXPECT_EQ("02", opts.ToHex());
+  EXPECT_TRUE(opts.HasStrictSpans());
 }
 
 TEST(TraceOptionsTest, Comparison) {
diff --git a/opencensus/trace/span.h b/opencensus/trace/span.h
index ae2c2629..31539a63 100644
--- a/opencensus/trace/span.h
+++ b/opencensus/trace/span.h
@@ -54,8 +54,9 @@ using AttributesRef =
 // Options for Starting a Span.
 struct StartSpanOptions {
   StartSpanOptions(Sampler* sampler = nullptr,  // Default Sampler.
-                   const std::vector<Span*>& parent_links = {})
-      : sampler(sampler), parent_links(parent_links) {}
+                   const std::vector<Span*>& parent_links = {},
+                   TraceOptions trace_options = {})
+     : sampler(sampler), parent_links(parent_links), trace_options(trace_options) {}
 
   // The Sampler to use. It must remain valid for the duration of the
   // StartSpan() call. If nullptr, use the default Sampler from TraceConfig.
@@ -67,6 +68,10 @@ struct StartSpanOptions {
   // Pointers to Spans in *other Traces* that are parents of this Span. They
   // must remain valid for the duration of the StartSpan() call.
   const std::vector<Span*> parent_links;
+
+  // Any additional trace options for this span, mostly useful if this is a
+  // new root span.
+  const TraceOptions trace_options;
 };
 
 // Span represents an operation. A Trace consists of one or more Spans.
@@ -150,6 +155,10 @@ class Span final {
   void AddChildLink(const SpanContext& child_ctx,
                     AttributesRef attributes = {}) const;
 
+  // Increments an internal counter of open child spans of this span, only
+  // used if StrictSpans is enabled on this trace
+  void IncrementChildCount() const;
+
   // Sets the status of the Span. See status_code.h for canonical codes.
   void SetStatus(StatusCode canonical_code,
                  absl::string_view message = "") const;
@@ -158,6 +167,11 @@ class Span final {
   // End is called.
   void End() const;
 
+  // Called to end a Span with given id. No further changes can be made to the
+  // Span after calling this. This is called in StrictSpans mode because a
+  // child span ends and needs to End() its parent and move it around the maps.
+  static void EndSpanById(const SpanId& id);
+
   // Returns the SpanContext associated with this Span.
   const SpanContext& context() const;
 
diff --git a/opencensus/trace/span_id.h b/opencensus/trace/span_id.h
index 8571cd0c..8e95bbb1 100644
--- a/opencensus/trace/span_id.h
+++ b/opencensus/trace/span_id.h
@@ -42,6 +42,9 @@ class SpanId final {
   // Returns false if the SpanId is all zeros.
   bool IsValid() const;
 
+  static_assert(kSize == 8, "Hash Function assumes 64 bit spanIds");
+  std::size_t hash() const;
+
   // Copies the opaque SpanId data to a buffer, which must hold kSize bytes.
   void CopyTo(uint8_t* buf) const;
 
diff --git a/opencensus/trace/trace_options.h b/opencensus/trace/trace_options.h
index 38956e01..c3d22a67 100644
--- a/opencensus/trace/trace_options.h
+++ b/opencensus/trace/trace_options.h
@@ -42,6 +42,10 @@ class TraceOptions final {
   // the process, e.g. to Stackdriver/Zipkin.
   bool IsSampled() const;
 
+  // HasStrictSpans means that a parent span cannot end until all of its
+  // children end.
+  bool HasStrictSpans() const;
+
   bool operator==(const TraceOptions& that) const;
 
   // Returns a 2-char hex string of the TraceOptions value.
@@ -54,6 +58,10 @@ class TraceOptions final {
   // is_sampled.
   TraceOptions WithSampling(bool is_sampled) const;
 
+  // Returns a copy of these TraceOptions with the strict_spans bit set to
+  // strict_spans.
+  TraceOptions WithStrictSpans(bool strict_spans) const;
+
  private:
   uint8_t rep_[kSize];
 };