This book provides a concise overview of the physical basics of different forms of renewable energy (water, waves, wind, solar, thermal, geothermal, biofuels), focusing on the physical limits for the efficiency and energy densities of different current technologies. It also discusses relevant aspects of materials science, physical chemistry, and biophysics. The book is based on the lecture notes of a course taught at TU München to undergraduate and graduate students of Applied Physics and related engineering disciplines. It provides material that can be taught in a one-semester course with 4 hours per week and includes a self-test section to enable students to check their understanding.