Chlamydia trachomatis is a prevalent pathogen in human populations in both developing and developed areas of the globe. Disease burden is heavy, since both acute and chronic complications can arise from infection. Emerging knowledge of the immune response to infection suggests that many of the complications of chlamydial infection are accompanied by important alterations in immunoregulation; new information suggests that both antibody-mediated and cell-mediated immune effectors may be significant in eliminating or limiting chlamydial infection. Further studies of the pathophysiologic mechanisms of host defense and the ways in which C. trachomatis escapes these defenses are needed. Elucidation of the immune response may provide the information necessary to redevelop a vaccine approach to disease control.