This book is primarily written from the perspective of the connection between cosmology and the physics of the micro-world. It presents findings related to the theory of cosmological perturbations, inflationary theory, and the theory of post-inflationary reheating. This work serves as a continuation of the monograph "Introduction to the Theory of the Early Universe: The Hot Big Bang Theory" (Moscow: URSS, 2008), which discussed results pertaining to a homogeneous isotropic universe during its hot evolutionary stage and subsequent cosmological phases. In this book, both established concepts of the early and modern universe are explored, alongside the most advanced and well-founded theoretical models that have yet to find experimental validation. Several sections employ methods from non-equilibrium statistical physics and more specialized techniques from quantum field theory. To facilitate the reading of these sections, necessary information is provided in the appendices. This book is intended for researchers, graduate students, and undergraduates specializing in elementary particle physics and cosmology.