As children, many of us remember going on a family outing to a zoo, an aquarium, a planetarium, or a natural history museum. Although sometimes we may have approached such excursions warily, thinking they might prove boring, eventually there was something that caught our eye. Perhaps it was a chimpanzee staring back at us in a strangely familiar way or a shark taking a solitary swim in a custom-made tank. It could have been a moon rock brought back to Earth from one of the first manned space flights. When, at the end of the outing, parents asked, “Did you have fun?” in spite of ourselves we usually had to say yes. But then they wanted to know something else: “What did you learn?” That question was far harder to answer. Indeed, those working in science museums and other informal learning environments, including film and broadcast media; botanical gardens and nature centers; libraries; and youth, community, and out-of-school-time programs, increasingly are being called on to answer this question. Although people have participated in these activities for at least 200 years, only in the past few decades have practitioners and evaluators in the informal science community begun to study systematically what people learn, how they learn, and whether experiences in informal environments reinforce people’s identity as science learners. This work, still in its early stages, has proven to be challenging for several reasons.