Chlamydia is an obligate intracellular bacterium with a biphasic life cycle.The chlamydial ability to replicate and survive inside host cells for long periods of time and to induce persistent inflammatory responses significantly contributes to the chlamydial pathogenicity.Chlamydial urogenital infection in women can lead to severe complications including pelvic inflammatory diseases, ectopic pregnancy and infertility.We have used the state-of-the-art approaches including the genome scale proteome array to unravel the molecular secrets of chlamydial intracellular life.We have found that Chlamydia has evolved multiple strategies for manipulating host signaling pathways.On one hand, Chlamydia can exploit the host cells for taking up nutrients and energy from host cells, and on the other, Chlamydia has to maintain the integrity of host cells and protect the infected cells from host defense mechanisms.To achieve these goals, Chlamydia secretes multiple virulence factors into the host cells.Identification of these virulence factors and understanding the mechanisms of their actions should provide essential information for improving diagnosis, searching for novel therapeutic targets, and developing effective subunit vaccines.