This document summarizes the role of calcium and calmodulin in cellular signaling. It discusses how calcium acts as a regulatory second messenger by binding to proteins like calmodulin. Calmodulin has four calcium-binding sites that allow it to adopt an open confirmation and bind target sequences. Calmodulin regulates enzymes like calcium/calmodulin dependent protein kinases, myosin light chain kinases, and calcineurin. These enzymes play important roles in processes like muscle contraction, transcription, and neuronal signaling by phosphorylating or dephosphorylating target proteins in response to calcium signals.