Chlamydia is a common Sexually Transmitted Infection (STI) that often has no symptoms so is not recognised and treated. Untreated Chlamydia can have serious reproductive health consequences in women and because of this we have a National Chlamydia Screening Programme. There are still many unanswered questions about risk factors for Chlamydia, influences on participation in screening and long term health effects of infection. Participants in the ALSPAC TF4 clinic were offered Chlamydia testing within the National Screening Programme. This found that infection was less common than in previous surveys but that infection was much more common in people experiencing social disadvantage. We want to study whether this is still true at age 24 and also lay the groundwork for later studies looking at long term effects of Chlamydia. We also want to add information from Chlamydia tests in the Age 24 clinic to other information we have on infection over the lifetime from previous tests, questionnaires and blood tests to build a full picture of lifetime infection with Chlamydia and the effects of this on health.