The general definitions of the finite element method can today be made so wide (vide Chapter 3) as to include other useful approximation processes. In particular finite difference methods will now be recognized as a subclass of the procedure and (with some imagination) the boundary integral method, which has been lately used with much success for certain classes of problem, can be brought under the general definition. This generalization is made with two-fold object. First to improve our understanding - second to incorporate selective advantages of the alternatives in a unified manner. Chapter 23 is devoted to a recent development by which the boundary integral and finite element methodologies are combined.