Many of the ideas discussed in this chapter have evolved from work at the author's lab. The author is especially indebted to his doctoral students over the years who have helped to provide a constant atmosphere of stimulating discussion. Several completed doctoral dissertations which are correlated to this chapter, along with references to tagged publications in English, are given in the following list of citations: M. Bodden [33,35,34,28], J.-P. Col [72,29], H. Els [90,91,92], W. Gaik [112,114,113], H. Hudde [144,146,145], H. Lehnert [179,180,181,28], U. Letens [184,32], W. Lindemann [189,190,191], W. Pompetzki [275], Ch. Pösselt [276,306], D. Schlichthärle [298], J. Schröter [304,30,305], H. Slatky [314], S. Wolf [365], N. Xiang [372,371,373].