This article describes the rationale for, and development of, an online instrument that helps identify commonly held science misconceptions. Science Beliefs is a 47-item instrument that targets topics in chemistry, physics, biology, earth science, and astronomy. It utilizes a true or false, along with a written-explanation, format. The true or false responses provide a cursory view of the extent to which specific beliefs are prevalent, while the accompanying explanations reveal underlying reasons for those beliefs. The stages of instrument development, reliability and validity information, along with the original sources of the items are discussed. The developed instrument has the potential to help science educators understand some specific barriers to deepening understanding across a range of science topics.