This chapter deals with the process of work hardening in aluminium and a number of its alloys. The approach taken is to develop a simple one parameter model based on dislocation density and modify this to try and describe the essential behaviour of a range of solid solution and precipitation hardening alloys. Attention is also given to large strain behaviour and its description. The basic framework for the description of work hardening is then related to formability and to the treatment of a number of commercial aluminium alloys.