Gravity currents and intrusions are the ubiquitous phenomena where a fluid of one density flows horizontally into a fluid of a different density. For researchers and engineers, the ability to understand and predict these flow fields is essential in wide-ranging applications including those involved with atmospheric and ocean dynamics, the propagation of pollutants, and the dispersion of volcanic clouds. Although many new theories, experimental data, simulation results, and insights have surfaced recently, most of these developments remain scattered in journals and conference papers. A systematic and unified introduction to the field, which incorporates the new results and developments, is bound to facilitate the application of the available knowledge to both practical problems and further investigations. Written by a recognized authority active in recent developments in gravity current research, An Introduction to Gravity Currents and Intrusions provides an up-to-date, self-contained, and systematic introduction to the study, interpretation, and prediction of gravity current flows. The author uses a simple mathematical framework to provide an accessible and comprehensive introduction to the mathematical and physical aspects of this important topic. The text supplies researchers and practitioners with the foundation required to formulate problems, interpret experimental results and measurements data, obtain simple and insightful analytical estimates and solutions, and develop or implement numerical codes for related problems. The book also points out gaps of knowledge in the current understanding that require further research. This accessible reference requires only a basic background in fluid mechanics and applied mathematics, making it an ideal starting place for researchers and engineers new to the field. It can also serve as a textbook for upper-level undergraduate and graduate level courses in fluid dynamics.