HIV-specific T cell immune responses will play an important role in any HIV vaccine paradigm. Studies in rhesus monkeys have shown that significant and persistent virus-specific T cell responses can be elicited with vaccines incorporating viral genetic sequences and that these responses are primarily mediated by CD8 T cells. Benefits such as stable CD4 levels and viral control have resulted. Two vaccine candidates developed by Merck and Co., Inc., including a non-replicating adenoviral vector, have been studied in animals and are now being studied in Phase I clinical trials in humans. Important considerations include cross-clade reactivity (effectiveness in diverse HIV-infected populations), tolerability, and durability of response. Ongoing studies are looking at responses in both uninfected and infected individuals. Optimal vaccine combinations as well as the development and testing of vaccines with multiple genetic targets are part of future plans investigating this vaccine strategy.