 Current HIV treatment guidelines recommend the construction of combination regimens comprising a minimum of three agents from at least two drug classes. There are problems with the current recommendations for although treatments are effective, their success is often limited by tolerability, adverse effects and the need to take many pills. Antiretroviral adherence remains vital and regimens should be simplified wherever possible to facilitate maximal adherence. The recent availability of the potent HIV integrase inhibitor, raltegravir, provides an opportunity to explore moves away from current regimen components. Evidence to support the use of novel regimens must be generated through adequately powered randomized clinical trials. However, before such trials can be undertaken, preliminary data to define the pharmacokinetics, safety and tolerability of these regimens are needed to minimize unnecessary risk for participants. This eight week study will investigate the steady-state pharmacokinetics, and short-term safety and efficacy of two dosing strategies once and twice daily of raltegravir plus atazanavir in treatment experienced HIV-infected adults. 