diff --git a/unified_planning/io/pddl_writer.py b/unified_planning/io/pddl_writer.py
index 5247ac951..b471016d6 100644
--- a/unified_planning/io/pddl_writer.py
+++ b/unified_planning/io/pddl_writer.py
@@ -1238,6 +1238,8 @@ def _write_derivative(
         out.write(f" (increase {fluent} (* #t {converter.convert(simplified_value)} ))")
     elif effect.is_decrease():
         out.write(f" (decrease {fluent} (* #t {converter.convert(simplified_value)} ))")
+    elif effect.is_derivative():
+        out.write(f" (increase {fluent} (* #t {converter.convert(simplified_value)} ))")
     else:
         raise UPProblemDefinitionError(
             "Derivative can only be expressed as increase, decrease in processes",
diff --git a/unified_planning/model/action.py b/unified_planning/model/action.py
index f0ec258b6..ab01f10cf 100644
--- a/unified_planning/model/action.py
+++ b/unified_planning/model/action.py
@@ -948,7 +948,7 @@ def add_derivative(
                 fluent_exp,
                 value_exp,
                 condition_exp,
-                kind=up.model.effect.EffectKind.INCREASE,
+                kind=up.model.effect.EffectKind.DERIVATIVE,
                 forall=tuple(),
             )
         )
diff --git a/unified_planning/model/effect.py b/unified_planning/model/effect.py
index 071160e22..2d7fed516 100644
--- a/unified_planning/model/effect.py
+++ b/unified_planning/model/effect.py
@@ -38,11 +38,13 @@ class EffectKind(Enum):
     `ASSIGN`   => `if C then F <= V`
     `INCREASE` => `if C then F <= F + V`
     `DECREASE` => `if C then F <= F - V`
+    `DERIVATIVE` => `dF/dt <= V`
     """
 
     ASSIGN = auto()
     INCREASE = auto()
     DECREASE = auto()
+    DERIVATIVE = auto()
 
 
 class Effect:
@@ -109,13 +111,16 @@ def __repr__(self) -> str:
             s.append(f"forall {', '.join(str(v) for v in self._forall)}")
         if self.is_conditional():
             s.append(f"if {str(self._condition)} then")
-        s.append(f"{str(self._fluent)}")
+        if not(self.is_derivative()):
+            s.append(f"{str(self._fluent)}")
         if self.is_assignment():
             s.append(":=")
         elif self.is_increase():
             s.append("+=")
         elif self.is_decrease():
             s.append("-=")
+        elif self.is_derivative():
+            s.append(f"d{str(self._fluent)}/dt =")
         s.append(f"{str(self._value)}")
         return " ".join(s)
 
@@ -244,6 +249,9 @@ def is_increase(self) -> bool:
     def is_decrease(self) -> bool:
         """Returns `True` if the :func:`kind <unified_planning.model.Effect.kind>` of this `Effect` is a `decrease`, `False` otherwise."""
         return self._kind == EffectKind.DECREASE
+    def is_derivative(self) -> bool:
+        """Returns `True` if the :func:`kind <unified_planning.model.Effect.kind>` of this `Effect` is a `derivative`, `False` otherwise."""
+        return self._kind == EffectKind.DERIVATIVE
 
 
 class SimulatedEffect:
diff --git a/unified_planning/test/test_model.py b/unified_planning/test/test_model.py
index a5b8582a7..6e9d23a2e 100644
--- a/unified_planning/test/test_model.py
+++ b/unified_planning/test/test_model.py
@@ -111,6 +111,36 @@ def test_clone_effect(self):
         self.assertNotEqual(e, e.value)
         self.assertNotEqual(e.value, e)
 
+    def test_process(self):
+        Vehicle = UserType("Vehicle")
+        a = Fluent("a",BoolType())
+        x = Fluent("x", IntType())
+        move = Process("moving",car = Vehicle)
+        move.add_precondition(a)
+        move.add_derivative(x,1)
+        e = Effect(
+            FluentExp(x), Int(1), TRUE(), unified_planning.model.EffectKind.DERIVATIVE
+        )
+        self.assertEqual(move.effects[0],e)
+        self.assertEqual(move.name,"moving")
+        self.assertEqual(isinstance(move,Process),True)
+        
+    def test_event(self):
+        Vehicle = UserType("Vehicle")
+        a = Fluent("a",BoolType())
+        x = Fluent("x", IntType())
+        fell = Event("fell",car = Vehicle)
+        fell.add_precondition(a)
+        fell.add_effect(x,1)
+        e = Effect(
+            FluentExp(x), Int(1), TRUE(), unified_planning.model.EffectKind.ASSIGN
+        )
+        self.assertEqual(fell.effects[0],e)
+        self.assertEqual(fell.name,"fell")
+        self.assertEqual(isinstance(fell,Event),True)
+        self.assertEqual(isinstance(fell,InstantaneousAction),False)      
+  
+
     def test_istantaneous_action(self):
         Location = UserType("Location")
         move = InstantaneousAction("move", l_from=Location, l_to=Location)