Many nonlinear problems in physics, engineering, biology, and social sciences can be reduced to finding critical points of functionals. While minimax and Morse theories provide answers to many situations and problems on the existence of multiple critical points of a functional, they often cannot provide much-needed additional properties of these critical points. Sign-changing critical point theory has emerged as a new area of rich research on critical points of a differentiable functional with important applications to nonlinear elliptic PDEs. Key features of this book: * Self-contained in-depth treatment of sign-changing critical point theory * Further explorations in minimax and Morse theories * Topics devoted to linking and nodal solutions, the sign-changing saddle point theory, the generalized Brezis–Nirenberg critical point theorem, the parameter dependence of sign-changing critical points * Applications of sign-changing critical point theory studied within the classical symmetric mountain pass theorem *Applies sign-changing concepts to Schrödinger equations and boundary value problems This book is intended for advanced graduate students and researchers involved in sign-changing critical point theory, PDEs, global analysis, and nonlinear functional analysis. Also by the author: (with Martin Schechter) Critical Point Theory and Its Applications, ©2006, Springer, ISBN: 978-0-387-32965-9.