This book provides a comprehensive review of the soft X-rays emitted when the solar wind interacts with exospheric neutrals at Venus, Mars, comets, the Moon, and Earth. It shows how observations can be used to address research problems ranging from the nature of magnetic reconnection at the Earth's magnetopause to atmospheric loss at other planets. The book provides the theoretical basis for soft X-ray emission and describes simulations of the expected emissions, past observations by narrow field-of-view X-ray telescopes, and current efforts to develop a new generation of wide field-of-view telescopes capable of capturing the entire solar wind-obstacle interaction.