Public trust in science is crucial for accepting scientific advances and following science-based recommendations. Equally, the study of public trust in science is crucial for identifying variations and understanding how and why laypeople gain or lose trust. This chapter addresses definition and measurement issues surrounding trust in science. First, trust is a frequently studied yet rarely defined variable. Second, trust in science and distrust in science are insufficiently differentiated from each other. Finally, available measurements of trust are often imprecise or not validated. Despite the growing interest in public and individual perceptions of science, more attention must be paid to precisely conceptualising and measuring trust and distrust in science. This chapter provides definitions of trust and distrust in science and presents two perspectives: (a) trust and distrust in science are one variable on a single continuum; and (b) trust and distrust in science are two variables on two separate continua. We then discuss measurement issues and provide recommendations for selecting the most appropriate measurement.