This book is intended to serve as a bridge for connecting the traditional continuum mechanics principles with modern computational procedures.It aims to convey that nonlinear analysis is not a direct extension or repeated application of linear analysis. In conducting a nonlinear analysis, each of the phases involved has to be qualified by certain rules, before the entire procedure can be put into work. To this end, the authors have proposed the rigid body rules for testing the nonlinear theories and associated finite elements derived