Toxoplasmosis is a major and preventable cause of severe visual handicap and blindness in young people. It represents the most frequent cause of posterior uveitis in Europe. Toxomplasma gondii is an obligate intracellular parasite and its definitive host is the cat. Oocysts which are excreted in the cat feces are very resistant and may remain infective for more than 1 year. The infected cat is very important in the contamination of environments (contaminated soil, fruit and vegetables and, thereby, also intermediate hosts, i.e., infected animals). The prevalence of toxoplasmic infection varies among different populations depending on socioeconomic factors, climate, eating habits and other factors. This chapter covers the congenital versus postnatally acquired infection, pathogenesis, clinical manifestations, diagnosis, and therapy. The diagnosis of ocular toxoplasmosis is mainly clinical; serologic tests are positive for a considerable percentage of the general population and are not indicative for ocular involvement. The importance of serologic testing in a given situation depends on the age of patient and the probability of attracting acquired infection from the environment.