Acute and persistent virus infections are limited by the development and maintenance of host T-cell responses to viral antigens. Individuals with congenital or acquired immunodeficiencies are at risk of progressive and often life-threatening infection. Recent studies have provided insight into the nature of protective T-cell responses to viruses and advances in T-cell culture technology have made it possible to evaluate the adoptive transfer of T-cell clones of defined antigen specificity and function to restore deficient responses in immunocompromised hosts. The progress of these studies in cytomegalovirus and human immunodeficiency virus infection is the subject of this review.