In order for a herbicide to kill plants, the chemical must be absorbed and translocated to the site of action. In the case of the imidazolinone herbicides, the site of action is the acetohydroxyacid synthase (AHAS) enzyme, which is concentrated in plant meristematic tissues. Absorption of the imidazolinones by roots and shoots and the subsequent translocation to the meristems is presented in this chapter following a brief review of the morphology and physiology of the transport system.