Immunocytochemistry exploits the incomparable specificity of the antibody-antigen interaction to form the basis of a flexible approach to the study of expression and localization of proteins both in model systems and their physiological context. This chapter details the theory and practice of the technique as well as lists the materials required. A general protocol is proposed, which can be adapted to suit the needs of individual investigators using the suggestions outlined in the Notes. The use of frozen tissue sections and cultured cells is described. Finally, the most common causes for failure of the technique are presented, along with likely solutions.