In this chapter, we introduce the concept of energy and energy conservation through two examples: A vertical bowshot and an atom moving along a surface. Based on the examples, we introduce the concept of potential energy for a position-dependent force to complement the kinetic energy. For objects subject only to position-dependent forces, the sum of the potential and kinetic energy is constant. We can therefore interpret a motion as a transfer of energy between kinetic and potential energies. We show how to calculate the potential energy for a constant force, a spring force, and a general position-dependent force, and how to use energy conservation to solve mechanics problems. We introduce the energy diagram as an alternative way to analyze and understand motion. We generalize the concept of potential energy to two- and three-dimensional motion. Finally, we introduce the general energy principle, the second law of thermodynamics, and relate external work to changes in the total energy of a system.