This document is a lecture on shear flow and shear stresses in beams and thin-walled members. It begins by defining shear flow in prismatic beams and relating it to the shear force and first moment of area. Examples are then given to calculate shear stresses in different beam cross-sections. The document extends these concepts to beams with arbitrary cross-sections and thin-walled members like flanges. It provides an example calculation of shear stress in the flange of a wide-flange beam. Diagrams and equations are presented throughout to illustrate the relationships between shear flow, shear stress, shear force, and moment of area.