Spontaneous change of type is a widely observed phenomenon in physics. In this volume, leading experts survey from a mathematical point of view topics such as phase transitions in crystals, cluster dynamics, viscoelastic flows, motion of interfaces in thermodynamics, shocks in transonic flows, and nonlinear diffusion with finite speed of propagation. Owing to new mathematical techniques, there is now a renewed interest in these difficult questions. The present volume supplies new results but may also serve as an excellent introduction to recent literature. It will be of interest to researchers and to graduate students in physics and mathematics.