Assembled in this volume are articles chosen from Methods in Enzymology Volumes 200 and 201 for their contemporary relevance and their usefulness to those new to the study of protein phosphorylation. Major topics include classification of protein kinases and protein phosphatases, assays of protein kinases and protein phosphorylation, purification and cloning of protein kinases and protein phosphatases, and the use of inhibitors of protein kinases and protein phosphatases. Key Features* Classification of Protein Kinases and Protein Phosphatases* Assays of Protein Kinases and Protein Phosphorylation* Purification and Cloning of Protein Kinases and Protein Phosphatases* Analysis and Identification of Phosphopeptides and Phosphoamino Acids