This book provides an overview of the evolution of ideas related to the concept of energy, emphasizing the observations, experiments, and discussions that led to the final definition of energy as a physical quantity and the formulation of the law of conservation of energy within the context of the mechanical theory of heat. Attention is focused on the observations and experiments conducted throughout history, with references to the original texts where they were reported. Many of these experiments can be repeated with low-cost, simple instruments and can serve as the basis for discussion in teaching, preparing students for the path toward the currently accepted laws. Justo Roberto Pérez Cruz (Villa de Mazo, 1960 ) is a Professor of Applied Physics at La Laguna University (Tenerife, Canary Islands, Spain). His main research has focused on Molecular Spectroscopy, and he has extensive experience in this field. He has taught thermodynamics, computational physics, the history and communication of physics, the physics of energy and the environment, and the physics of sports, and is the author of several textbooks on these subjects. He has served as Dean of the Faculty of Physics and Deputy Vice Chancellor of La Laguna University, and he has also led several projects in popular culture and scientific dissemination.