Few bacterial pathogens are as widespread in nature or as capable of eliciting such a diversity of disease syndromes as are the chlamydiae. As obligate intracellular organisms, they pose a special research challenge in defining the molecular components and mechanisms for productive growth within host cells and the overall progress of infection throughout host tissue. Although a comprehensive view of chlamydial envelope composition and respective functions in pathogenesis is far from complete, ongoing investigations continue to expose new and intriguing avenues for exploration.