This book provides a pedagogical introduction to the rapidly growing field of reheating after inflation. It begins with a brief review of the inflationary paradigm and a motivation for why the reheating of the universe is an integral part of inflationary cosmology. It then goes on to survey different aspects of reheating in a chronological manner, starting from the young, empty and cold universe at the end of inflation, and going all the way to the hot and thermal universe at the beginning of the Big Bang nucleosynthesis epoch. Different particle production mechanisms are considered with a focus on the non-perturbative excitation of scalar fields at the beginning of reheating (fermionic and vector fields are also discussed). This is followed by a review of the subsequent non-linear dynamical processes, such as soliton formation and relativistic turbulence. Various thermalization processes are also discussed. High energy physics embeddings of phenomenological models as well as observational implications of reheating such as gravitational waves generation and imprints on the cosmic microwave background are also covered.