Calcium/calmodulin-dependent protein kinase II (CaMKII) is a ubiquitous enzyme, present in essentially every tissue but most concentrated in brain. It is a multifunctional Ca2+/calmodulindependent protein kinase capable of phosphorylating protein substrates, such as AMPA receptors, synapsin I, tyrosine hydroxylase, L-type Ca2+ channels, and MAP-2 (microtubuleassociated protein 2), substrates that are found in essentially every intracellular compartment (Hudmon and Schulman, 2002; Lisman et al., 2002; Colbran and Brown, 2004). Proper function of the kinase requires that it access and selectively phosphorylate numerous substrates in diverse intracellular sites. This review will discuss the role of kinase localization in optimizing fidelity of substrate phosphorylation.