Bringing together eleven essays by the distinguished philosopher of science. Peter Achinstein, Particles and Waves explores the nature of the philosophical problems surrounding the postulation of unobservable entities such as light waves, molecules, and electrons. Achinstein examines these problems as they arose in actual scientific practice in three nineteenth-century episodes: the debate between particle and wave theorists of light, James Clerk Maxwell's kinetic theory of gases, and J.J. Thomson's discovery of the electron.