Presents an integrated account of modern developments in energy methods for studying free boundary problems in PDEs. The theory presented has particular relevance to a number of physical applications, including heat conduction, surface and underground water flow, filtration through porous media, flows of non-Newtonian fluids, boundary layers, chemical reactions, and semiconductors. The work is divided into two parts: An exposition of the methods of several general classes of nonlinear equations and systems and applications to the theory. Of interest to applied mathematicians and graduate students whose research is in partial differential equations, nonlinear analysis, and continuum mechanics. Applications to a number of different problems arising in continuum mechanics (fluid dynamics) are presented making this book of equal interest to physicists and engineers.