This introduction to the special issue Understanding the Public Understanding of Science: Psychological Approaches discusses some of the challenges people face in understanding science. We focus on people's inevitably bounded understanding of science topics; research must address how people make decisions in science domains such as health and medicine without having the deep and extensive understanding that is characteristic of domain experts. The articles reflect two broad streams of research on the public understanding of science-the learning orientation that seeks to improve understanding through better instruction and the communications orientation that focuses on attitudes about science and trust in scientists. Challenges to understanding science include determining the relevance of information, the tentativeness of scientific truth, distinguishing between scientific and nonscientific issues, and determining what is true and what is false. Studying the public understanding of science can potentially contribute to psychological theories of thinking and reasoning in modern societies.