How well we manage long-term environmental risks depends on how well we understand them. Whether the risk managers are experts or laypeople, that understanding is typically limited. As a result, people must rely on judgment when making decisions about risks. Estimating how big risks are and how much reducing them is worth is an intellectual skill. After reviewing the behavioral principles that govern how people acquire such skills, this article offers several proposals for facilitating learning abut risks by improving the ways in which scientific data are created or presented. It also describes some pitfalls facing attempts to determine the quality of other people's understanding of risks, whether through direct study or more casual observation. © 1990 Kluwer Academic Publishers.