The advancement of quantum mechanics has given physics a completely new direction from that of classical physics in the early days. In fact, there is a constant development in this subject of a very fundamental nature, such as implications for the foundations of physics, physics of entanglement, geometric phases, gravity and cosmology and elementary particles as well. This book will be an important resource for researchers with respect to present topics of research in this developing area. The book covers two important sections: Perturbation Theory and Foundations of Quantum Mechanics.