Why and how have lay people participated in scientific observation? And on what terms have they collaborated with experts and professionals? We have become accustomed to the involvement of lay observers in the practice of many branches of science, including both the natural and human sciences, usually as subordinates to experts. The current surge of interest in this phenomenon, as well as in the closely related topic of how expertise has been constructed, suggests that historians of science can offer a valuable contribution to these vital questions. A historical approach to lay participation allows us to better understand the making of expert-lay relations in science, and it offers a broader, long-term perspective on contemporary debates about that boundary.