Introduction to the fundamental processes governing atmospheric circulations and climate. Starting from an overview of the observed state of the atmosphere and its variation over the past, the course discusses Earth's radiative energy balance including the greenhouse effect, Earth's orbit around the Sun and climatic effects of its variations, and the role of atmospheric circulations in maintaining the energy, angular momentum, and water balances, which determine the distributions of temperatures, winds, and precipitation. The focus throughout is on order-of-magnitude physics that is applicable to climates generally, including those of Earth's past and future and of other planets.