This lecture covers the concepts of laminar, transitional, and turbulent flow in pipes, including the Reynolds experiment, limits of each flow regime, entrance region, fully developed flow, pressure distribution, shear stress distribution, and the Hagen-Poiseuille law. It also discusses the relation of pressure drop to velocity, fully developed laminar flow, flow in non-horizontal pipes, friction factors, Colebrook's formula, and equivalent roughness for new pipes.