In the most general sense, an accelerator is any device designed to give kinetic energy to ions or electrons. According to this definition the earliest cathode ray and canal ray tubes, made before the turn of the century were accelerators, the same principle is used in the x-ray tube, the cathode ray oscilloscope, the mass spectrograph, the electron microscope, and many other modern devices. However, we shall limit this discussion to those accelerators made for the particular purpose of inducing nuclear reactions, and to a few others that may be of interest in this connection, although they were originally designed for other purposes, such as high-voltage x-ray tubes intended for deep therapy or the radiography of metals. Although much work related to accelerators is thus omitted, it must not be forgotten that the principles learned and the practical experience gained by this work were of great importance in the development of the machiens now used in nuclear physics.