The law of conservation of energy is one of the most fruitful results to emerge from Newtonian mechanics. Since its origin, the law has been extended to include thermal and electromagnetic energy and forms of nuclear energy. Physically, energy is identified with the capacity to perform work. This chapter discusses potential energy to enlarge the law of conservation of energy. A system possessing potential energy is also capable of performing work. The chapter presents the notion of a closed path to lay the foundation for a definition of potential energy. A closed path may be of arbitrary shape but its beginning and ending must be at the same point. The chapter distinguishes between two types of forces—conservative and non-conservative. It explains the conservation principle of kinetic energy. If only conservative forces act, then the total energy of the particle is conserved.