Infection with the bacterium, Chlamydia trachomatis (often termed "chlamydia") is the most frequently reported sexually transmitted infection in the United States. The urethra is the most common site of infection in males, and the urethra and cervix are most commonly infected in females. Ascending infection in females can cause pelvic inflammatory disease, which can lead to infertility and ectopic pregnancy. Genital chlamydial infections are usually asymptomatic, and screening is necessary to identify most infections. Currently, chlamydia screening for sexually active women aged <25 years is recommended by the U.S. Preventive Services Task Force (grade B recommendation). Chlamydia is nationally notifiable; however, if females do not access care or clinicians do not screen, many infections go undiagnosed, unreported, and untreated. CDC monitors population prevalence of genital chlamydial infection through the National Health and Nutrition Examination Survey (NHANES), which tests a sample of the U.S. population aged 14-39 years for genital C. trachomatis and found that the overall chlamydia burden in the United States decreased during 1999-2008. Using data from the most recent cycles of NHANES (2007-2012), CDC estimated chlamydia prevalence among persons aged 14-39 years overall and by demographic characteristics and sexual behaviors. The prevalence of chlamydia among persons aged 14-39 years was 1.7% (95% confidence interval [CI] = 1.4%-2.0%). Chlamydia prevalence varied by age and race/ethnicity, with prevalence highest among non-Hispanic blacks (5.2%). Among sexually active females aged 14-24 years, the population targeted for routine screening, chlamydia prevalence was 4.7% overall and 13.5% among non-Hispanic black females. As chlamydia is common and infections are usually asymptomatic, health care providers should routinely screen sexually active young women aged <25 years for chlamydial infection, provide prompt treatment for infected persons, and ensure that infected patients' sex partners receive timely treatment to prevent reinfection.