Theoretical physics is the study of simple and complex natural phenomena occurring in the universe using abstraction and mathematical models. There are myriad areas of research in theoretical physics that are actively being pursued in the world today like string theory, chaos theory, astrophysics, cosmology and biophysics. The pillars of modern theoretical physics are relativity theory and quantum mechanics. These help in understanding astronomical, cosmological phenomena as well as phenomena at the particle level. This book traces the progress of this field and highlights some of its key concepts and applications. Researchers and students in this field will find it a valuable text to stay updated with the latest developments across all areas of theoretical physics. It unfolds the innovative aspects of this field, which will be crucial for the progress of this discipline in the future.