With the purpose of promoting cooperative research involving the fields of mechanics and pure mathematics, the International Society for the Interaction of Mechanics and Mathematics (ISIMM) sponsors a series of Symposia. The ninth in this series (STAMM 94) took place in July 1994 at the University of Lisbon and emphasized the current trends in nonlinear mechanics, phase change problems (in cooperation with the European Science Foundation Scientific Programme on Mathematical Treatment of Free Boundary Problems), non Newtonian fluids, optimization in solid mechanics and numerical methods in continuum mechanics. This book collects a refereed selection of original contributions presented at STAMM 94, covering a large spectrum of current research in the above topics, from nonlinear elasticity to nonlinear fluids, from phase transitions to diffusion phenomena, and from structural optimization and homogenization to numerical schemes.