This book provides a concise and accessible exposition of a wide range of topics in geometric approaches to differential equations. The aim of the book is to present an overview of this developing subject and a brief introduction to a number of related topics, including twistor theory, vortex filament dynamics, calculus of variations, exterior differential systems and Bäcklund transformations. Written by leading experts, this book is an ideal starting point for graduate students embarking on research. It will also be of use to researchers and anybody wishing to learn more about this burgeoning field of mathematical endeavour.