The knowledge of the physiological role of calcium has increased considerably in the past years. A new perspective was introduced with the isolation of a calcium binding protein, calmodulin, which is ubiquitous for every tissue and organ of all eukaryotes. Beside their prominent role in muscle contraction calcium ions were found to play a major role in signal transduction within the cell. It has been reported that calcium ions initiate many physiological phenomena, such as cell division, fertilization, platelet aggregation, and neurotransmitter biosynthesis. Calcium is now regarded as a second messenger together with cAMP and inositoltrisphosphate. Main topics covered are - calcium mobilization in membranes, - calcium receptors and calmodulin-dependent enzymes, and - calcium signals and medical problems.