In most of vertebrates, mature oocytes arrest at the metaphase of the II meiotic division, while some invertebrates arrest at metaphase-I, others at prophase-I. Fertilization induces completion of meiosis and entry into the first mitotic division. Several experimental models have been considered from both vertebrates and invertebrates in order to shed light on the peculiar aspects of meiotic division, such as the regulation of the cytostatic factor (CSF) and the maturation promoting factor (MPF) in metaphase I or II. Here, we reviewed the role of CSF and MPF and their biochemical pathways in regulating meiosis completion. Differences and similarities existing within several model systems of invertebrates, such as ascidians, cnidarians, mollusks, starfish, will be analyzed and compared to meiotic regulation in Xenopus. Data will be analyzed at the light of the phylogenetic conservation of MPF and CSF functions, accordingly to the position of these organisms in the evolutionary tree.