This book explores some of the complex ethical quandaries entailed by proposed phase III HIV preventive vaccine trials. The book argues that such trials must be initiated as soon as politically and ethically feasible on the one hand, and that no such trials should be undertaken until we can assure full compliance with the Nuremberg Code and the WHO/CIOMS International Ethical Guidelines for Biomedical Research Involving Human Subjects on the other. The tension between these two positions is fully detailed and suggestions offered for how to think about possible resolutions.