The last few decades have seen considerable growth in the role played by amateurs in the sciences. With the development of new techniques for collecting information, new virtual networks and the emergence of new problematics calling for the participation of citizens, this role has also become more visible, while the modern boundary between professionalism and amateurism, first erected in the 19th century, has been shaken. These contemporary developments have changed our perspective on amateurs in science and brought forth questions and analyses that sometimes coincide with recent inflections in the history of science. Thus it is now possible to take a new approach to the historical study of amateurs in contemporary science. This introduction hopes to demonstrate this, while the essays brought together in this volume, some of which explore extreme cases, reveal the very relative nature of the definition of the “amateur” category and how complex and fertile its implementation has been in the history of science.