Theoretical physics is a cornerstone of modern physics and provides a foundation for all modern quantitative science. It aims to describe all natural phenomena using mathematical theories and models and, in consequence, develops our understanding of the fundamental nature of the universe. This book offers an overview of major areas covering the recent developments in modern theoretical physics. Each chapter introduces a new key topic, and develops the discussion in a self-contained manner. At the same time, the selected topics have common themes running throughout the book, which connect the independent discussions. The main themes—renormalization group, fixed points, universality and continuum limit—open and conclude the work. Other important and related themes are path integrals and field integrals, effective field theories, gauge theories, the mathematical structure at the basis of the interactions in fundamental particle physics, including quantization problems and anomalies, stochastic dynamical equations and summation of perturbative series.