This book describes various S-FEM models in a systematic, concise and easy to understand manner. The underline principles, theories, techniques for shape functions, means for strain field modification, formulation procedures, and implementation strategies will be presented in detail. A large number of examples will be provided to demonstrate the efficiency and accuracy of the S-FEM solutions in comparison with the FEM. Practical techniques for computing both upper and lower bounds of solutions using both S-FEM and FEM will be presented. The book presents also a systematically way to obtain solutions of desired accuracy using a discrete model.