Inositol trisphosphate (InsP3) functions as a second messenger to control the release of internal calcium and the entry of external calcium. This InsP3/Ca2+ signalling pathway is based on a hierachical system with the release from individual channels being the fundamental event (Ca2+ blips). A collection of blips produces larger elementary events (Ca2+ puffs) which then fuse through a regenerative process of calcium-induced calcium release to give global events (Ca2+ waves). This highly versatile signalling system has been adapted to regulate a wide range of cellular processes including fertilization, development, proliferation and apoptosis.