Members of the genus Chlamydia cause a plethora of ocular, genital and respiratory diseases, with severe complications, such as blinding trachoma, pelvic inflammatory disease, ectopic pregnancy and tubal factor infertility, interstitial pneumonia, and chronic diseases that may include atherosclerosis, multiple sclerosis, adult-onset asthma and Alzheimer's disease. The current medical opinion is that an effective prophylactic vaccine would constitute the best approach to protect the human population from the most severe consequences of these infections. There are three essential and mutually inclusive areas of challenge confronting researchers developing Chlamydia vaccines. These are to define the elements of protective immunity and the basis of vaccine evaluation, the judicious selection of an immunogenic and safe antigen(s) to form the basis of a subunit vaccine, and to develop effective delivery systems that boost the immune response to achieve long-lasting protective immunity. The development of delivery vehicles and adjuvants to boost protective long-term immunity against chlamydiae currently poses the greatest challenge in vaccine research. However, enormous progress is being made in the construction of novel delivery systems, such as DNA and plasmid expression systems, viral vectors, and living and non-living bacterial delivery systems, and the use of chemical adjuvants. In addition, there is increasing effort being made in designing delivery strategies involving specific immunomodulatory procedures that modify the cytokine and chemokine environment, upregulate co-stimulatory molecules and target vaccines to specific mucosal sites. These efforts will likely culminate in an efficacious chlamydial vaccine in the near future.