The principals of four methods for measurements of viral load (HIV endpoint dilution, NASBA, bDNA and Amplicor PCR) are outlined. HIV quantification has increased our knowledge about the natural history of the HIV infection and the influences of exogenous factors on HIV replication. Furthermore measurements of HIV RNA early in the course of the infection have proven to be prognostic for the clinical outcome of the infection. Last but not least, HIV quantification has been shown to be a good marker for the efficacy of antiretroviral treatment and will hence be a valuable tool for the monitoring and management of anti-retroviral treatment.