Calmodulin, a Ca²⁺ receptor, is an important constituent of cellular signal transduction systems. In recent years, much progress has been made on the three-dimensional structure of calmodulin, this leads the scientists to understand the mechanisms of the activation of calmodulin by Ca²⁺ and the interaction between calmodulin and its target enzymes. A concise description is given on the 3D structures of Apo-calmodulin, Ca²⁺-calmodulin and the complex of calmodulin with its target peptides and antagonists.