The HIV/AIDS epidemic continues to grow rapidly, throughout the world. This is likely to remain so because i) HIV cannot be cured, ii) prevention is neglected and iii) a preventive vaccine remains elusive. In contrast, on the therapeutics front there has been tremendous progress. Since 1996, highly active antiretroviral therapy (HAART) has transformed AIDS into a chronic manageable illness. HAART stops HIV replication, therefore turning plasma viral levels to undetectable levels (<50 c/mL). This leads to a sustained immune recovery and near normal health. HAART has also been shown to decrease HIV levels in semen and vaginal secretions. As a result there has been growing interest regarding the possible role of HAART in directly preventing HIV transmission by decreasing infectiveness of those on treatment. Supportive evidence for "HAART as Prevention" first emerged in vertical transmission studies where, ultimately, HIV transmission from the infected mother to her child has become exceedingly rare if HAART is in place. A possible effect of HAART on decreasing HIV transmission has been incidentally observed in HIV serodiscordant heterosexual couples and separately in population-level studies from Taiwan, San Francisco and in our own study in British Columbia, Canada. We therefore propose to prospectively test the "HAART as Prevention" hypothesis, with a special focus on IDU. Specifically we propose to test that: 1) expanded HAART coverage among IDU will lead to a decrease in the number of new HIV infections within the population, including but not restricted to IDU;and 2) expanded HAART coverage among IDU, will lead to a decrease in adverse HIV/AIDS health outcomes among IDU. This proposal, named "Seek and Treat for Optimal Outcomes and Prevention in HIV &AIDS in IDU", represents both a groundbreaking and innovative research hypothesis with important consequences for the control of the global HIV epidemic.