The science of superconductivity is expanding speedily and new inventions are possible in the near future. This book is a compilation of researches accomplished by experts dealing with a variety of theoretical aspects of superconductivity such as eilenberger approach, cooper pairing, theory of ferromagnetic, etc. The authors have attempted to express their unique vision and give an insight into the examined area of research. We intend to help our readers in understanding the above stated topic in a better way.