This award-winning monograph addresses perturbation methods in critical point theory. It particularly emphasizes applications such as semilinear elliptic problems on Rⁿ, bifurcation from the essential spectrum, the prescribed scalar curvature problem, nonlinear Schrödinger equations, and singularly perturbed elliptic problems in domains. Its discussion of the abstract tool of perturbation methods in critical point theory is not available elsewhere. The book was awarded the Ferran Sunyer i Balaguer Prize 2005.