Providing exact solutions of more than 200 nonlinear equations and models, this book begins with classical as well as more recent examples of interesting solutions on linear invariant subspaces for nonlinear operators. In the remainder of the book, the authors develop several techniques for constructing exact solutions that describe singularity behavior for various nonlinear PDEs, including gas dynamics models, free-boundary problems, Green-Naghdi equations, and quasilinear pseudo-parabolic (magma) equations. Using exact solutions, they describe the evolution properties related to singularity blow-up or extinction phenomena, finite interface propagation and regularity, and the oscillatory, changing sign behavior of weak solutions near interfaces.