This lecture provides an introduction to the physics of nuclear reactors, covering the history, objectives, and main goals of the course. It includes a review of nuclear physics, discussing the composition of a nucleus, nuclear radii, nuclear mass, binding energy, mass defect, binding energy per nucleon curve, and nuclear reactions. The lecture also explores concepts such as magic nuclei, excited states in nuclei, and radioactivity, explaining the spontaneous disintegration of nuclei and radioactive decay. Conservation laws in nuclear reactions and a summary of key points are also presented.