Boosted protease inhibitor regimens combine ritonavir with a second, ‘boosted’ protease inhibitor to enhance patient exposure to the latter agent, thereby preventing or overcoming resistance and allowing less frequent dosing, potentially improving adherence. The advantages offered by ritonavir boosting are primarily attributable to the drug’s pharmacokinetic properties. Ritonavir’s inhibition of the cytochrome P-450 CYP3A4 enzyme reduces the metabolism of concomitantly administered protease inhibitors and changes their pharmacokinetic parameters, including area under the curve (AUC), maximum concentration (C_max), minimum concentration (C_min) and half-life (t_1/2). As a result, the bioavailability of the boosted protease inhibitor is increased and improved penetration into HIV reservoirs may be achieved. Boosted protease inhibitor regimens that utilize a low dose of ritonavir (100–200 mg) appear to offer the best balance of efficacy and tolerability. At this dose, ritonavir boosts the bioavailability of the second protease inhibitor without contributing significantly to the side effect profile of the regimen. In clinical trials, regimens boosted with low dose ritonavir have demonstrated high levels of viral suppression in both antiretroviral naïve patients and patients who previously failed antiretroviral therapy, including protease inhibitor therapy. Side effects observed have generally been similar to those associated with the boosted protease inhibitor. Based upon their enhanced drug exposure and demonstrated efficacy, the boosted ritonavir regimens should be among the first options considered for use in clinical practice.