The purpose of the present protocol is to derive a comprehensive description of HIV population genetics in a longitudinal, observational study of HIV-infected patients prior to initiating antiretroviral therapy. We plan to utilize frequent blood sampling and an extensive sequencing strategy to investigate parameters of HIV population genetics, including: a) genotypic and phenotypic analyses of HIV drug resistance mutations, b) determinations of the rates at which mutations arise, become fixed, lost, or undergo recombination, c) linkage analyses, d) estimates of the size of the effective virus population. We plan to apply this information to develop models of HIV evolution, predict the genetic behavior of HIV populations, including the emergence of resistant genomes. We expect that information regarding HIV population genetics may assist in designing appropriate drug regimens to salvage control of HIV virus replication after initial regimens have failed.