Perhaps the most complex point in any traffic network is the signalized intersection. At a typical signalized intersection with four entry approaches on two two-way streets, there are 12 vehicular movements (4 approaches, each with a left-turn, through, and right-turn movement), four crosswalk pedestrian flows, and bicycles that must be taken into consideration. The intricacies of signal timing have a significant impact on operations, as does the physical design of each approach. Moreover, the “intersection” does not operate as a whole: each approach can have different demands and experience different operating conditions. The development of capacity and level of service analysis methodologies for signalized intersections reflects the complexity of operations in the field, and the number of underlying factors that influence them. This chapter details signalized intersection analysis methodologies from their beginnings in the 1950 HCM through the 1980 Interim Procedures for Highway Capacity.