Infections by the protozoan parasite Toxoplasma gondii are widely prevalent in humans and other animals on all continents. This chapter provides a history of the milestones in the acquisition of knowledge of the biology of this parasite. Toxoplasmosis in sheep deserves special attention because of its economic impact. The identification of T. gondii abortion in ewes is considered a landmark discovery in veterinary medicine; prior to that, protozoa were not recognized as a cause of epidemic abortion in livestock. The ability to identify T. gondii infections based on a simple serological test opened the door for extensive epidemiological studies on the incidence of infection. It became clear that T. gondii infections are widely prevalent in humans in many countries. It also demonstrated that the so-called tetrad of clinical signs considered indicative of clinical congenital toxoplasmosis occurred in other diseases and assisted in the differential diagnosis. Vaccination of sheep with a live cystless strain of T. gondii reduces neonatal mortality in lambs, and the vaccine is available commercially.