This chapter discusses the mechanism of enzyme regulation by calmodulin and Ca2+. The interactions between Ca2+, calmodulin, and a calmodulin-dependent enzyme can be described by a set of macroscopic equilibria. Information on the various interactions between Ca2+, calmodulin, and calmodulin-dependent enzymes is of particular importance as these interactions ultimately dictate the nature of the Ca2+ dependence of these enzymes. Quantitative characterization of these interactions has been hindered by the fact that the interaction between calmodulin and its target enzymes in the absence of Ca2+ is usually of very low affinity, and the Ca2+ binding to the enzyme-calmodulin complex cannot be determined directly. The activity of PDE as a function of Ca2+ and calmodulin can be measured with high precision by using the technique of flow-microcalorimetry to monitor the PDE-catalyzed hydrolysis of cAMP. A possible structural change on calmodulin when it binds to PDE is the reorientation of the α-helices flanking the Ca2+-binding loop. This would lead to a change in the position of the amino acid residues that coordinate with Ca2+ and thus change the affinity of Ca2+. Recent NMR studies indicate that the positions of the α-helices in calmodulin are altered when the protein is bound with Ca2+; whether the helices undergo further reorientation with enzyme binding remains to be determined.