Immunocytochemistry is a highly productive method in biomedical research used to identify proteins and other macromolecules in tissues and cells. Control samples are required to show label localization is correct, but the understanding and use of immunocytochemistry controls have been inconsistent. A new classification of immunocytochemical controls is proposed that will help in understanding this most important component of the experiment. The three types of controls required for immunocytochemistry are primary antibody controls that show the specificity of the primary antibody binding to the antigen, secondary antibody controls that show the label is specific to the primary antibody, and label controls that show the labeling is the result of the label added and not the result of endogenous labeling. Publications containing immunocytochemical results must give details of how these controls were performed.