During the last 20 years the study of, and the prediction of, changes in the climate of our planet have become an urgent social imperative, addressed to scientists the world over. The first principles on which to base such a study were formulated in 1974 in Skholm, at the international GARP conference on the physical fundamentals of climate theory and climate modeling. In 1979 the World Meteorological Organization and the International Council of Scientific Unions decided to conduct a global program of climate research. This World Climate Program is designed mainly to investigate the variability of the climate on time scales ranging from a few weeks to a few decades and to create a scientific basis for the long-term forecasting of weather. There is at present a definite need for a monograph which can serve as an introduction to the theory of climate. On a qualitative level (without the apparatus of theoretical physics and mathematics) such an introduction has already been presented, in Part I of a book on the history of climate by Yu. A. Shishkov and the author (Monin and Shishkov, 1979). Part II of that work gives factual data on climatic changes during the course of the Earth's history. The present book is designed to provide such an introduction on a quantitative level.