Technical Abstract: This is a brief review of the effects of chicken anemia virus (CAV) on the immune system of chickens. CAV was first isolated in Japan as a contaminant in a Marek's disease virus vaccine and was first isolated in the U.S. about ten years later. CAV is a member of the circovirus family, which is highly stable in the environment (non-enveloped) has a DNA genome and is very small (20-23nm). CAV has a worldwide distribution and very high incidence; it is likely that all commercially produced chickens are exposed to CAV at some point during their lifetimes. The primary importance of CAV in commercially produced chickens is due to the immunosuppression it causes in young chickens. Immunosuppression can adversely affect production characteristics including decreased weight gain, increased unevenness, poor response to vaccination and increased condemnations due to secondary infection.