The mechanisms underlying the Ca (2+) release at fertilization of several animal organisms are reported. Four main classical theories are described, i.e., that of Ca (2+) release following simple sperm contact and a G protein stimulation; that of simple sperm contact followed by a tyrosine kinase receptor activation that of the necessity of introduction by sperm into the egg of molecules for Ca (2+) release; and that the molecule introduced into the marine eggs for Ca (2+) release is the same Ca (2+). Two other mechanisms for Ca (2+) release are also illustrated: that of ryanodine receptor stimulation and that of NAADP formation. (c) 2005 Elsevier Inc. All rights reserved.