A report commissioned by the European Centre for Disease Prevention and Control (ECDC), coordinated by Otilia Sfetcu and Marita van de Laar ; Executive summary The literature reviews in this report bring together published evidence about the prevalence and reproductive tract complications of chlamydia infection, and about the effectiveness and cost-effectiveness of chlamydia screening interventions. Population prevalence of chlamydia in EU/EEA Member States • Ten EU/EEA Member States have conducted population-based cross-sectional surveys to measure the prevalence of chlamydia infection in a nationally representative sample of the population or in a subnational sample of the population. Fourteen EU/EEA Member States have conducted cross-sectional surveys using non-population-based sampling methods. Three EU/EEA Member States have no studies estimating chlamydia positivity or prevalence. • Estimates of chlamydia prevalence in population-based studies varied by country, sex, age group, national or sub-national coverage and inclusion of all or only sexually experienced participants. • Four EU/EEA Member States (France, Germany, Slovenia, UK) have reported findings from nationally representative surveys of sexually experienced adults ≤25 years, with response rates from 46 to 71%. Chlamydia point prevalence estimates in women aged 15–24 years ranged from 3.0% (18–24 year olds in UK) to 4.7% (18–24 year olds in Slovenia). Point prevalence estimates in men aged 15–24 years ranged from 0.4% (16–17 year olds in Germany) to 4.7% (18–24 year olds in Slovenia). Estimates of chlamydia prevalence in EU/EEA Member States were statistically consistent with those in other high income countries. • Selection bias in chlamydia prevalence surveys is likely, with over-estimation of prevalence being more likely than under-estimation. Cross-sectional surveys with lower response rates are associated with higher estimates of chlamydia prevalence. Estimates of chlamydia positivity in surveys with low response rates should not be interpreted as estimates of population prevalence. Only two population-based surveys in EU/EEA Member States in this review had a response rate of >70%. The highest response rates were seen when specimens for chlamydia testing were taken as part of general health surveys. • Reporting standards for prevalence surveys in epidemiological research might help to improve consistency in future.