diff --git a/PhysLean.lean b/PhysLean.lean
index 283c464e5..160955282 100644
--- a/PhysLean.lean
+++ b/PhysLean.lean
@@ -41,6 +41,10 @@ import PhysLean.Electromagnetism.PointParticle.ThreeDimension
 import PhysLean.Electromagnetism.Vacuum.Constant
 import PhysLean.Electromagnetism.Vacuum.HarmonicWave
 import PhysLean.Electromagnetism.Vacuum.IsPlaneWave
+import PhysLean.FluidMechanics.IdealFluid.Basic
+import PhysLean.FluidMechanics.IdealFluid.Bernoulli
+import PhysLean.FluidMechanics.IdealFluid.Continuity
+import PhysLean.FluidMechanics.IdealFluid.Euler
 import PhysLean.Mathematics.Calculus.AdjFDeriv
 import PhysLean.Mathematics.Calculus.Divergence
 import PhysLean.Mathematics.DataStructures.FourTree.Basic
diff --git a/PhysLean/FluidMechanics/IdealFluid/Basic.lean b/PhysLean/FluidMechanics/IdealFluid/Basic.lean
new file mode 100644
index 000000000..9fc4840a0
--- /dev/null
+++ b/PhysLean/FluidMechanics/IdealFluid/Basic.lean
@@ -0,0 +1,94 @@
+/-
+Copyright (c) 2026 Michał Mogielnicki. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Michał Mogielnicki
+-/
+
+import Mathlib.Analysis.InnerProductSpace.PiL2
+import Mathlib.Analysis.Calculus.ContDiff.Basic
+import PhysLean.SpaceAndTime.Space.Basic
+import PhysLean.SpaceAndTime.Time.Basic
+
+/-!
+This module introduces the idea of an ideal fluid and the mass flux density
+and basic physical properties, meant to be later used for proofs.
+-/
+
+open scoped InnerProductSpace
+
+/- Introducing the structure of Ideal Fluids -/
+structure IdealFluid where
+  /- The density at a specific point and time -/
+  density: Time → Space → ℝ
+  /- The velocity at a specific point and time -/
+  velocity: Time → Space → EuclideanSpace ℝ (Fin 3)
+  /- The pressure at a specific point and time -/
+  pressure: Time → Space → ℝ
+  /- The entropy at a specific point and time -/
+  entropy: Time → Space → ℝ
+  /- The enthlapy at a specific point and time -/
+  enthlapy: Time → Space → ℝ
+
+  density_pos: ∀ t pos, 0 < density t pos
+
+  /- Ensuring that all are differentiable for more complex equations. -/
+  density_contdiff: ContDiff ℝ 1 ( fun(s:Time × Space)=>density s.1 s.2)
+  velocity_contdiff: ContDiff ℝ 1 ( fun(s:Time × Space)=>velocity s.1 s.2)
+  pressure_contdiff: ContDiff ℝ 1 ( fun(s:Time × Space)=>pressure s.1 s.2)
+
+  entropy_contdiff: ContDiff ℝ 1 ( fun(s:Time × Space)=>entropy s.1 s.2)
+  enthlapy_contdiff: ContDiff ℝ 1 ( fun(s:Time × Space)=>enthlapy s.1 s.2)
+
+namespace IdealFluid
+open MeasureTheory
+
+/- Mass flux density j=ρv -/
+def massFluxDensity (F: IdealFluid) (t: Time) (pos: Space):
+    EuclideanSpace ℝ (Fin 3) :=
+     (IdealFluid.density F t pos) • (IdealFluid.velocity F t pos)
+
+/- Entropy flux density ρsv -/
+def entropyFluxDensity (F: IdealFluid) (t: Time) (pos: Space):
+    EuclideanSpace ℝ (Fin 3) :=
+      (IdealFluid.entropy F t pos) • (IdealFluid.density F t pos) • (IdealFluid.velocity F t pos)
+
+/- Energy flux density ρv(1/2 |v|^2 + w) -/
+noncomputable def energyFluxDensity (F: IdealFluid) (t: Time) (pos: Space):
+    EuclideanSpace ℝ (Fin 3) :=
+      let w := IdealFluid.enthlapy F t pos
+      let v := IdealFluid.velocity F t pos
+      let v_sq: ℝ := ⟪v,v⟫_ℝ
+      let scalar := (IdealFluid.density F t pos)*(0.5*v_sq + w)
+
+      scalar • v
+
+/- Volume to introduce surface integrals -/
+structure FluidVolume where
+  /- The 3D region -/
+  region: Set Space
+  /- The normal pointing outwards -/
+  normal: Space → EuclideanSpace ℝ (Fin 3)
+  /- 2D measure of the boundary -/
+  surfaceMeasure: Measure Space
+
+/- Surface integral of a vector field -/
+noncomputable def surfaceIntegral (V: FluidVolume) (flux: Space → EuclideanSpace ℝ (Fin 3)):
+    ℝ :=
+      ∫ (pos: Space) in frontier V.region, ⟪flux pos, V.normal pos⟫_ℝ ∂V.surfaceMeasure
+
+/- Mass flow out of volume -/
+noncomputable def massFlowOut (F: IdealFluid) (t: Time) (V: FluidVolume):
+    ℝ :=
+      surfaceIntegral V (fun pos => IdealFluid.massFluxDensity F t pos)
+
+/- Entropy flow out of volume -/
+noncomputable def entropyFlowOut (F: IdealFluid) (t: Time) (V: FluidVolume):
+    ℝ :=
+      surfaceIntegral V (fun pos => IdealFluid.entropyFluxDensity F t pos)
+
+/- Energy flow out of volume -/
+noncomputable def energyFlowOut (F: IdealFluid) (t: Time) (V: FluidVolume):
+    ℝ :=
+      surfaceIntegral V (fun pos => IdealFluid.energyFluxDensity F t pos)
+
+end IdealFluid
diff --git a/PhysLean/FluidMechanics/IdealFluid/Bernoulli.lean b/PhysLean/FluidMechanics/IdealFluid/Bernoulli.lean
new file mode 100644
index 000000000..a9245b215
--- /dev/null
+++ b/PhysLean/FluidMechanics/IdealFluid/Bernoulli.lean
@@ -0,0 +1,34 @@
+/-
+Copyright (c) 2026 Michał Mogielnicki. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Michał Mogielnicki
+-/
+
+import PhysLean.FluidMechanics.IdealFluid.Basic
+import PhysLean.Mathematics.Calculus.Divergence
+
+/-
+This module introduces:
+steady flow,
+... (still under construction)
+-/
+
+open scoped InnerProductSpace
+open Time
+open Space
+
+namespace IdealFluid
+
+/- Determines whether a flow is steady -/
+def isSteady (F: IdealFluid) :
+    Prop :=
+      ∀ (t : Time) (pos : Space), ∂ₜ (fun t' => IdealFluid.velocity F t' pos) t = 0
+
+/- Determines whether a flow is isentropic -/
+def isIsentropic (F: IdealFluid):
+    Prop :=
+      ∀ (t: Time) (pos: Space), ∂ₜ (fun t' => IdealFluid.entropy F t' pos) t = 0
+
+-- TODO: Provide the Bernoulli's equation (after fun derivatoins)
+
+end IdealFluid
diff --git a/PhysLean/FluidMechanics/IdealFluid/Continuity.lean b/PhysLean/FluidMechanics/IdealFluid/Continuity.lean
new file mode 100644
index 000000000..9f41a2f9e
--- /dev/null
+++ b/PhysLean/FluidMechanics/IdealFluid/Continuity.lean
@@ -0,0 +1,32 @@
+/-
+Copyright (c) 2026 Michał Mogielnicki. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Michał Mogielnicki
+-/
+
+import PhysLean.FluidMechanics.IdealFluid.Basic
+import PhysLean.Mathematics.Calculus.Divergence
+
+/-
+This module introduces the continuity criterium.
+There is potential to add various different lemmas expanding on this.
+-/
+
+open scoped InnerProductSpace
+open Time
+open Space
+
+namespace IdealFluid
+
+/- defining satisfying the equation of continuity -/
+def satisfiesContinuity (F : IdealFluid):
+    Prop :=
+      ∀ (t : Time) (pos : Space),
+      ∂ₜ (fun t' => IdealFluid.density F t' pos) t +
+      Space.div (fun pos' => IdealFluid.massFluxDensity F t pos') pos = 0
+
+
+-- TODO: Add lemmas for continuity with different models.
+-- TODO: Add definition and lemmas for Incompressibility.
+
+end IdealFluid
diff --git a/PhysLean/FluidMechanics/IdealFluid/Euler.lean b/PhysLean/FluidMechanics/IdealFluid/Euler.lean
new file mode 100644
index 000000000..3901414b5
--- /dev/null
+++ b/PhysLean/FluidMechanics/IdealFluid/Euler.lean
@@ -0,0 +1,5 @@
+/-
+Copyright (c) 2026 Michał Mogielnicki. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Michał Mogielnicki
+-/