This paper reviews the optimization and control of thermal energy storage systems. Emphasis is given to thermal storage applied to combined heat and power systems, building systems, and solar thermal power systems. The paper also discusses how applications of thermal storage can benefit the chemical industry. Optimization of the design and control of thermal storage systems improves plant performance and improves the management of transient energy loads in a variety of applications. In order to maximize the benefits of thermal storage, it is necessary to include advanced multi-variate constrained controls, such as model predictive control. Thermal storage also increases system flexibility, allowing the incorporation of intermittent renewable energy sources. The flexibility of thermal storage will play an increasingly important role as utilities implement smart grid technology with time-of-use electricity pricing. Lastly, thermal energy storage improves system economics by reducing required equipment sizes, improving efficiency, and reducing equipment wear.