This text was developed over a five-year period during which its authors were teaching the subject. It is the culmination of successful editions of class notes and preliminary texts prepared for their one-semester course at MIT designed for sophomores majoring in physics but taken by students from other departments as well. The book describes the features that vibrations and waves of all sorts have in common and includes examples of mechanical, acoustical, and optical manifestations of these phenomena that unite various parts of physics. The main emphasis, however, is on the oscillatory aspects of the electromagnetic field—that is, on the vibrations, waves, radiation, and the interaction of electromagnetic waves with matter. The content is designed primarily for the use of second or third year students of physics who have had a semester of mechanics and a semester of electricity and magnetism. The aim throughout is to provide a mathematically unsophisticated treatment of the subject, but one that stresses modern applications of the principles involved. Descriptions of devices that embody such principles—such as seismometers, magnetrons, thermo-nuclear fusion experimental configurations, and lasers—are introduced at appropriate points in the text to illustrate the theoretical concepts. Many illustrations from astrophysics are also included.