Toxoplasma gondii is a ubiquitous protistan parasite of animals. Work on this parasite, and toxoplasmosis, the disease it causes, has been the subject of thousands of publications. A two-volume bibliography published in 1983 1 contained 12,476 references. Therefore, the number of publications on the biology, pathology, and immunology of T. gondii published at the time of writing this chapter must be in the tens of thousands. It is impossible to distill all available information into this chapter, accordingly this chapter concentrates on work in the major areas of interest of T. gondii research published during the last 5 years and thus remains an enormous task. Research carried out prior to 1982 on pathology and immunology was well covered earlier. 2