The use of drug resistance testing to guide the selection of an antiretroviral regimen for patients in whom current therapy is failing has gained growing acceptance in clinical practice. Genotypic and phenotypic resistance testing has been associated with improved short-term virologic outcome in prospective interventional trials. There is also growing evidence that monitoring drug levels, particularly of PIs, may add to the benefit provided by resistance testing. This study will assess the impact of TDM and resistance testing on lowering viral load in treatment-experienced patients and will also evaluate the mean change in plasma HIV RNA from study entry to Step 2 of the study.