This chapter is about measuring instantaneous power, active power, reactive power, power factor, and energy. We characterize different types of power system components based on their power profiles. Measuring energy is covered based on fixed intervals and fixed increments, as well as net energy metering and feed-in energy metering. We discuss smart meters and their applications in price-based and incentive-based demand response programs, as well as in baseline calculation in demand response applications. Other applications of smart meter measurements are covered, such as in load profiling and load classification. We discuss load disaggregation, net load disaggregation, and sub-metering, as well as static load modeling and dynamic load modeling. State estimation is also covered by using power measurements, including both nonlinear and linearized formulations. The following subjects are also discussed in this chapter: three-phase and unbalanced power measurements and their applications, accuracy in power and energy measurements, including different accuracy classes, meter accuracy versus system accuracy, as well as different factors that affect accuracy.