Mammalian embryo development begins when the fertilizing sperm triggers a series of elevations in the oocyte’s intracellular free Ca²⁺ concentration. The elevations are the result of repeated release and re-uptake of Ca²⁺ stored in the smooth endoplasmic reticulum. Ca²⁺ release is primarily mediated by the phosphoinositide signaling system of the oocyte. The system is stimulated when the sperm causes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) into inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG); IP₃ then binds its receptor on the surface of the endoplasmic reticulum that induces Ca²⁺ release. The manner in which the sperm generates IP₃, the Ca²⁺ mobilizing second messenger, has been the subject of extensive research for a long time. The sperm factor hypothesis has eventually gained general acceptance, according to which it is a molecule from the sperm that diffuses into the ooplasm and stimulates the phosphoinositide cascade. Much evidence now indicates that the sperm-derived factor is phospholipase C-zeta (PLCζ) that cleaves PIP₂ and generates IP₃, eventually leading to oocyte activation. A recent addition to the candidate sperm factor list is the post-acrosomal sheath WW domain-binding protein (PAWP), whose role at fertilization is currently under debate. Ca²⁺ influx across the plasma membrane is also important as, in the absence of extracellular Ca²⁺, the oscillations run down prematurely. In pig oocytes, the influx that sustains the oscillations seems to be regulated by the filling status of the stores, whereas in the mouse other mechanisms might be involved. This work summarizes the current understanding of Ca²⁺ signaling in mammalian oocytes.