In the study of trachoma, animal models have been used extensively to show the spectrum of disease that chlamydiae can cause. Animal studies have shown that repeated episodes of reinfection are necessary for the development of a trachoma model. The recognition of the importance of reinfection has wide-ranging implications for clinical and epidemiologic studies. Models can also be used to study the pathophysiology of disease. Although some studies on the immune response to chlamydial infection have been performed, much further work could be done to clarify the apparently paradoxical nature of the response. The immune response seems to be partially responsible for both protection and the destructive consequences of chlamydial infection. Animal models will need to be exploited during the development of vaccines.