Extracellular signals, including neurotransmitters, hormones, cytokines, and growth factors, produce many of their physiological effects by regulating the state of phosphorylation of specific proteins in target cells (Walaas and Greengard, 1991). Inhibitors of protein kinases and protein phosphatases are useful tools in evaluating the physiological roles of protein phosphorylation signaling mechanisms, and may ultimately find therapeutic applications. Available inhibitors include natural and synthetic compounds, peptides, and proteins (MacKintosh and MacKintosh, 1994). Natural inhibitors include endogenous protein inhibitors of protein kinases (e.g., the protein kinase inhibitor [PKI] of cyclic AMP [cAMP]-dependent protein kinase [PKA]) and protein phosphatases (e.g., inhibitor-1, inhibitor-2, and DARPP-32), as well as toxins produced by bacteria, fungi, plants, dinoflagellates, and insects. Synthetic inhibitors include peptide analogs derived from intramolecular pseudosubstrate inhibitory domains of protein kinases and phosphatases, derivatives of natural toxins, and analogs and inhibitors of endogenous activators (e.g., cAMP analogs) or substrates (e.g., ATP analogs). This chapter will review the most widely available inhibitors with demonstrated utility, including their basic properties and applications relevant to their use in neuroscience research.