Highly Active Antiretroviral Therapy (HAART) has transformed the nature of HIV/AIDS from an imminent death sentence to a chronic manageable condition. There is a growing interest in the potential impact of “treatment as prevention” — a notion that expanded coverage with HAART would substantially reduce morbidity and mortality from HIV, and secondarily HIV transmission at the population level. However, undiagnosed and undocumented HIV cases cannot benefit from HAART and are more likely to experience HIV-related morbidity and untimely mortality. Late diagnosis of HIV remains prevalent and represents missed opportunities for early linkage to and engagement in care. In the absence of a vaccine and cure for HIV, undiagnosed HIV cases represent the biggest challenge yet in the fight against HIV/AIDS. As such, identifying them should be a priority of HIV prevention programs.