This paper reviews recent studies on the life cycle of Toxoplasma gondii. Tachyzoites, bradyzoites, and sporozoites are the three infectious stages of T. gondii. Humans and animals become infected mainly by ingesting bradyzoites or oocytes. After ingestion, both bradyzoites and sporozoites convert to tachyzoites inside tissues. The conversion of tachyzoites to bradyzoites and bradyzoites to tachyzoites is of biological and clinical significance because bradyzoites are less susceptible to chemotherapy and reactivation of bradyzoites to tachyzoites is considered the cause of fatal toxoplasmosis in AIDS patients. Of all the methods currently available to assess stage conversion of T. gondii, feeding infective stages to cats is the most reliable method. Felidae, the definitive hosts of T. gondii excrete oocysts 3-10 days after ingesting tissue cysts/bradyzoites, > or = 18 days after ingesting oocysts, and > or = 13 days after ingesting tachyzoites.