This book presents recent and cutting edge advances in our understanding of key aspects of the response of materials under extreme loads that take place during high velocity impact and penetration. The focus of the content is on the numerous challenges associated with characterization and modeling of complex interactions that occur during these highly dynamic events. The following specific topics, among others, are addressed: characterization of material behavior under extreme loadings (estimate of damage, effects related to moisture contents, large pressures, large strain rates, etc.); measurement of microstructural changes associated with damage and mesoscopic scale modeling; macroscopic modeling, using the framework of the theory of viscoplasticity and damage; modeling and simulation of localization, cracking, and dynamic fragmentation of materials; application to penetration mechanics and trajectory instabilities. The book gathers together selected papers based on work presented as invited lectures at the 2nd US-France symposium held on 28-30 May 2008 in Rocamadour, France. The conference was organized by Eric Buzaud (DGA, Centre d'Études de Gramat) under the auspices of the International Center for Applied Computational Mechanics (ICACM).