The major portion of this chapter will be devoted to a discussion of the “mixing length theory,” or “MLT,” of convective heat transport in stars. Although this theory has many faults, it has served as a useful phenomenological model for a description of stellar convection for more than thirty years. Almost all numerical simulations of stellar evolution use it in one guise or another. Near the end of the chapter we shall briefly discuss alternatives to the MLT and why a realistic description is so difficult.