Quality assurance (QA) is a systematic process to monitor and improve clinical laboratory practices. The fundamental components of a laboratory QA program include providing a functional and safe laboratory environment, trained and competent personnel, maintained equipment, adequate supplies and reagents, testing of appropriate specimens, internal monitoring of quality, accurate reporting, and external quality assessments. These components are necessary to provide accurate and precise CD4 T-cell counts, an essential test to evaluate start of and monitor effectiveness of antiretroviral therapy for HIV-infected patients. In recent years, CD4 testing has expanded dramatically in resource-limited settings. Information on a CD4 QA program as described in this article will provide guidelines not only for clinical laboratory staff but also for managers of programs responsible for supporting CD4 testing. All agencies involved in implementing CD4 testing must understand the needs of the laboratory and provide advocacy, guidance, and financial support to established CD4 testing sites and programs. This article describes and explains the procedures that must be put in place to provide reliable CD4 determinations in a variety of settings.