The disturbed state concept (DSC) is a relatively new development and until now has lacked extensive attention in a publication. This book presents the first detailed, theoretical treatment of the DSC and shows that it can provide a unified and simplified approach for mathematical characterization of the mechanical response of materials and interfaces. The author compares the DSC with other available models, identifies the physical meaning of the relevant parameters, validates the DSC models implements the model in computer procedures, and validates those procedures by comparing predictions with observations.