This chapter reassesses the importance of scientific interests and concepts in Jean-Jacques Rousseau's career. It argues that the conventional portrait of Rousseau as a dilettante and critic of science proceeds from a biased view of scientific activity based on the present style of science as a highly specialized activity of professional researchers. When contextualized against the background of the ways and manners of scientific practices in eighteenth-century France, Rousseau can be described as a typical amateur, who participated in a number of scientific networks. As printed culture proliferated during the Enlightenment, a large number of scientific treatises circulated, which claimed to provide elementary notions in mathematics, astronomy, physics, or chemistry in more or less academic or entertaining styles. Finally, the chapter shows that Rousseau's practice of science had a significant impact on his major works.