This study will last from about 3 years to 5 years, because the length of the study depends on how quickly people enroll and how quickly during the study new HIV-1 infections occur. Study participants will be randomly assigned to receive a total of three injections of the DNA vaccine VRC-HIVDNA016-00-VP followed by one injection of the adenovirus vaccine VRC-HIVADV014-00-VP, for a total of four injections of vaccine or four injections of placebo. Injections will occur at study entry and Weeks 4, 8, and 24. Prior to the study injections, participants will have their vital signs and weight measured, and blood collection will occur. Participants will be observed in the clinic for at least 30 minutes after each injection for immediate reactions to the vaccines. At all injection visits, HIV risk-reduction counseling, HIV risk assessment, pregnancy prevention counseling, and training on how to use memory cards will also occur. For 3 to 7 days after each injection, participants will be asked to record information about injection site pain, redness, size, swelling, temperature, general well-being, and headaches on their memory cards. Additional study visits will occur on Weeks 1, 12, 28, 48, 72, 96, 120, and 144. At these visits, physical examinations, blood collection, and social impact questionnaires may also be done. Any study participants who become infected with HIV while on the study will be monitored for at least 72 weeks after diagnosis or at least 12 weeks after the study reaches its primary evaluation time point (whichever is longer). The study investigators are committed to providing access to local standard of care and treatment to those study participants who are found to be HIV-1 infected.