This special issue looks at the history of science as practiced by amateurs during the nineteenth and twentieth centuries. Our approach to these scientific activities privileges objects as material and visual sources. As readers will note from the articles, the topic is interdisciplinary in nature: first and foremost, because the extremely varied field of amateur hobbies appeals equally to historians of science, the arts, or sports, as well as to social and cultural historians. Amateurs themselves are versatile enthusiasts with a wide range of tastes. They may also assume different identities via their practice: as layman, a member of a learned society, an independent investigator, or as one challenging professional science. The history of scientific amateurism therefore extends the history of scientific production to socio-cultural dynamics that go beyond the boundaries of legitimate science.