This book provides an overview of the discovery of liquid water on Mars and the physical and geochemical processes that cause liquid saline water to form and stabilize in cold and dry environments such as the shallow martian subsurface and some areas of the polar region. The book also analyzes the implications of the discovery of liquid water for microbial life on Mars, the most Earth-like planet in the solar system.It explores the applications of scaling and thermodynamics to planetary sciences and explains how these concepts led to the discovery of liquid saline water on Mars. It covers the fundamental concepts necessary to understand the stability of liquid water in planetary environments.