Although Pneumocystis carinii pneumonia is one of the leading causes of morbidity and mortality among patients with the acquired immunodeficiency syndrome, many questions about its epidemiology and transmission remain unanswered. Whereas traditional theory postulates that the disease results from reactivation of latent infection, recent data suggest that active acquisition of infection, either through environmental exposure or person-to-person transmission, may occur. This review summarizes the current state of knowledge about the epidemiology and transmission of P. carinii and reports on evolving techniques that may improve our understanding of this organism in the future.