The luminosity of the sun governs the temperatures of the planets. Yet the solar forcing, or driving, of climate, primarily due to changes in solar radiation, has never been well documented. Recent satellite measurements have shown that solar radiation varies as a function of time and wavelength, a concept that has been hypothesized for the past two centuries and has recently become a major topic with all the attention paid to global warming. This book reviews the physics of the concept of solar forcing, from its beginnings in the early 1800's and apparent success in the 1870's, to its near demise in the 1950's and recent resurgence. Since its emphasis is on solar variations as a driver for climate change, with only a brief discussion of other mechanisms, the book will be of most interest to students in climate studies.