Written for the general reader, this book provides an overview of modern theoretical physics, focusing on the history and development of theoretical physics over the past century, while providing supplemental mathematical derivations for the more advanced reader. It tells the ongoing challenges to unify theories for the fundamental forces of nature, and reveals the interplay of the foundations of classical mechanics and Maxwell's equations, special and general relativity theory, quantum mechanics and quantum field theory, the standard model of particle physics, the theoretical framework of string theory, the cosmological theory for the evolution of the universe, and the theory of black holes. This subject matter brings forth the most interesting and revolutionary ideas in modern theoretical physics, taking the reader through a fascinating journey with a broad overview of the evolving universe that we live in.