AbstractThis overview provides a history of protein phosphorylation research and provides the reader with an understanding of how and why labeling studies are performed. The various sites of protein phosphorylation are described along with the roles of the many kinases and phosphatases that regulate phosphorylation. Methods for detecting unlabeled phosphoamino acids, including high‐voltage electrophoresis on thin‐layer cellulose acetate plates, gel‐shift assays, and the use of anti‐phosphopeptide antibodies.