Theoretical physics is a branch of physics that studies physical objects through mathematical methods. It is applied to study natural phenomena and also to predict and analyse it. This book on theoretical physics seeks to incorporate the various theories that have been developed into well-established systems as well as its implications on other fields. It discusses the fundamentals as well as modern approaches of theoretical physics. As this field is emerging at a rapid pace, the contents of this book will help the readers understand the modern concepts and applications of the subject. This text will be of help to students and teachers in the fields of quantum engineering, fluid theory and general relativity.