Serologic testing for hepatitis B surface antigen (HBsAg) is the primary way to identify persons with chronic hepatitis B virus (HBV) infection. Testing has been recommended previously for pregnant women, infants born to HBsAg-positive mothers, household contacts and sex partners of HBV-infected persons, persons born in countries with HBsAg prevalence of >/=8%, persons who are the source of blood or body fluid exposures that might warrant postexposure prophylaxis (e.g., needlestick injury to a health-care worker or sexual assault), and persons infected with human immunodeficiency virus. This report updates and expands previous CDC guidelines for HBsAg testing and includes new recommendations for public health evaluation and management for chronically infected persons and their contacts. Routine testing for HBsAg now is recommended for additional populations with HBsAg prevalence of >/=2%: persons born in geographic regions with HBsAg prevalence of >/=2%, men who have sex with men, and injection-drug users. Implementation of these recommendations will require expertise and resources to integrate HBsAg screening in prevention and care settings serving populations recommended for HBsAg testing. This report is intended to serve as a resource for public health officials, organizations, and health-care professionals involved in the development, delivery, and evaluation of prevention and clinical services.