The ability to quantitate human immunodeficiency virus (HIV) in blood and tissues from patients at all stages of disease has provided new insights into the pathogenesis of HIV disease. There is a dynamic equilibrium between HIV production and clearance even during the period of clinical latency, which may permit resistant virus to emerge with the imposition of drug pressure. Disruption of the equilibrium with effective drugs reduces circulating levels of HIV within 1 week, thus allowing the rapid assessment of new candidate drugs. To maximize the magnitude and durability of HIV RNA suppression, therapeutic strategies must be implemented that are effective against high levels of rapidly replicating virus that consist of many genetic variants.