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.