The paper presents the theory of micropolar fluids, a class of fluids that respond to micro-rotational motions and spin inertia, allowing them to support couple stress and distributed body couples. The theory is derived from the principles of continuum mechanics, incorporating conservation laws, balance of momentum, energy, and entropy. The equations of motion, constitutive equations, and boundary conditions are formulated for micropolar fluids, which are shown to be a subclass of microfluids. The theory is extended to include thermodynamic restrictions, leading to the derivation of field equations and boundary conditions. The paper also discusses the thermodynamics of micropolar fluids, establishing necessary and sufficient conditions for the local Clausius-Duhem inequality to be satisfied. The theory is applied to the problem of channel flow, where the equations are solved to demonstrate the existence of micropolar fluids. The results show that micropolar fluids can exhibit different behavior compared to classical fluids, such as lower skin friction due to micro-rotational effects. The paper concludes that the theory of micropolar fluids offers a valuable extension to fluid mechanics, with potential applications in various fluid types and new directions in turbulence theory. The theory is validated through numerical calculations and experimental data, showing its relevance and applicability in fluid dynamics.The paper presents the theory of micropolar fluids, a class of fluids that respond to micro-rotational motions and spin inertia, allowing them to support couple stress and distributed body couples. The theory is derived from the principles of continuum mechanics, incorporating conservation laws, balance of momentum, energy, and entropy. The equations of motion, constitutive equations, and boundary conditions are formulated for micropolar fluids, which are shown to be a subclass of microfluids. The theory is extended to include thermodynamic restrictions, leading to the derivation of field equations and boundary conditions. The paper also discusses the thermodynamics of micropolar fluids, establishing necessary and sufficient conditions for the local Clausius-Duhem inequality to be satisfied. The theory is applied to the problem of channel flow, where the equations are solved to demonstrate the existence of micropolar fluids. The results show that micropolar fluids can exhibit different behavior compared to classical fluids, such as lower skin friction due to micro-rotational effects. The paper concludes that the theory of micropolar fluids offers a valuable extension to fluid mechanics, with potential applications in various fluid types and new directions in turbulence theory. The theory is validated through numerical calculations and experimental data, showing its relevance and applicability in fluid dynamics.