Energy is the capacity to do work and is a conserved quantity, meaning it is not lost but merely converted into other forms. Familiar kinds of energy include thermal energy, potential energy, gravitational potential energy, chemical energy, and electromagnetic energy. The last is a result of electron flow in wires arranged in a circuit. The Ideal Gas Law can be used to show the relationship of the molecular energy of gaseous molecules and temperature, a key variable in energy analysis. The conversion of energy between kinds is usually analyzed by a control volume that encloses an energy transfer process and from which we can monitor flows of energy in and out. It is a powerful technique allowing the engineer to develop models of efficiency that are vital in the modern world. Key Words: conservation of energy, control volumes, thermal energy, potential energy, gravitational potential energy, chemical energy, electromagnetic energy.