A unifying theme among the herpesviruses is the intimate interrelationship of virus infection with host cellular immune competence. Herpesviruses suppress cellular immunity. Recent advances suggest mechanisms of herpesvirus-induced immunosuppression that include inhibition of cytokine production and direct interaction of virus with major histocompatibility complex (MHC) components. This suppression appears to be of clinical significance primarily in patients with underlying immune deficiencies.