In this chapter, viscous flow is discussed in detail. This kind of flow represents the most common flow in daily life and industrial production. Firstly, shearing motion and flow patterns of viscous Fluids is introduced, characteristics of laminar flow and turbulent flow is discussed. Secondly, Prandtl’s boundary-layer theory is introduced and boundary-layer equation is derived from the Navier-Stokes equation through dimensional analysis. Thirdly, some theory and facts for turbulent boundary layer are introduced. Fourthly, some shear flows other than boundary layer flow, such as pipe flow, jets, and wakes are briefly introduced. Boundary layer separation is the most important issue in engineering design, so it is introduced and discussed in a separate section in depth. The two top concerns, namely the flow drag and the flow losses are discussed in a separate section with examples and illustrations. Some further knowledge concerning turbulent flow is briefly discussed in the “expanded knowledge” section, such as the theory of homogeneous isotropic turbulent flow and the numerical computation of turbulent flows.