Until effective vaccines against human immunodeficiency virus (HIV) are avail able for general public use, the major public health strategy for limiting the spread of acquired immunodeficiency syndrome (AIDS) will be through educa tion about the risks of AIDS and instruction in strategies for reducing that risk. But developing effective educational programs requires a deeper understanding of how people think about the risks of AIDS and how they make decisions regarding those risks. Studies of risk perception and risk-taking behavior in other domains have identified a number of systematic biases in people’s risk percep tions and decision-making processes. These behavioral biases may lead people to make poor decisions that affect their risk of acquiring or spreading HIV infec tion. They also pose a significant obstacle to attempts to develop effective risk communications regarding AIDS and transmission of HIV.