The goal of this book is to promote the use of level set methods by the many scientists and engineers working on moving interface problems. The authors provide some motivational/intuitive background on the scope and variety of moving interface problems and their mathematical formulation. They also review the pros and cons of traditional numerical techniques. The bulk of the book addresses the foundation of essential mathematical and numerical methods necessary for applying the level set method, with particular emphasis on problems in which the interface is just one part of a more complicated physical system. The book concludes with a few select example applications drawn from the authors' research. The book is intended for students and researchers interested in computation in the physical sciences, i.e.engineers, computational fluid dynamicists and applied mathematicians who want to use these methods for their own computations. It is suitable for use in a graduate level course on numerical methods, as a users guide to applying the methods, and as a general reference for mathematics related to level sets and the numerical solution of equations of ''Hamiltonian-Jacobi'' type.