This book provides an introduction to degree theory and its applications to nonlinear differential equations. It uses an applications-oriented to address functional analysis, general topology and differential equations and offers a unified treatment of the classical Brouwer degree, the recently developed S?1-degree and the Dold-Ulrich degree for equivalent mappings and bifurcation problems. It integrates two seemingly disparate concepts, beginning with review material before shifting to classical theory and advanced application techniques.