Calmodulin (CaM) acts as an intracellular calcium sensor that translates the Ca2+ signal into a variety of cellular processes. Ca(2+)-CaM recognition of a short polypeptide segment in target proteins induces conformational changes in both CaM and the target, enabling the target protein to become functionally active. The solution and crystal structures of Ca(2+)-CaM bound to peptides derived from three CaM-dependent enzymes reveal structural features that are common in target recognition by Ca(2+)-CaM. Phosphorylation of the target proteins at sites in or near the CaM-binding region modulates binding of CaM, thereby providing an additional mechanism of functional regulation. The structural aspects of target recognition by Ca(2+)-CaM are discussed using mainly the three-dimensional structural information obtained with nuclear magnetic resonance spectroscopy and X-ray diffraction methods.