The quality of electric power depends on the power network topology, the amount of harmonic pollution injected in the network by nonlinear loads, and the severity of switching transients. Many of the loads encountered in modern power electronics, such as are and induction furnaces, welders, motor drives, and many types of converters, cause a significant level of harmonic pollution and/or recurrent voltage transients. This paper describes the major sources of disturbances that affect electric service quality and explains the indices that help quantify the severity of disturbances. The loads that are most sensitive to power quality are discussed, and techniques intended to avoid or mitigate power quality problems are detailed. Finally, a brief survey of the cost of harmonic pollution and consumer outages is presented.