This chapter examines two important strengthening mechanisms, martensitic and bainitic transformations, both of which occur under nonequilibrium cooling conditions. It explains how time-temperature-transformation diagrams are constructed and how they are used to understand and control the formation of martensite and bainite in steel and other alloys. It describes the morphology of both types of structures, the factors that influence their formation, how they respond to tempering processes, and their effect on mechanical properties and behaviors. It also discusses the role of transformation hysteresis in shape memory alloys.