This book is the result of a collaborative process. All the chapter contributors, except one, have been either guided or guiding by Prof S. Mangi Singh for the award of the degree of Doctor of Philosophy. It is apparent from the various chapters that the coverage of the book is very profound and extensive. It in fact shows the wide range of research carried out under his guidance. Why we said this is because eight out of the nine chapters are either abstracted or summarized either from the PhD thesis for which the degree has already been awarded or are still in progress.