During the past several years, demand for electric power has been increasing at the same time that electricity supply margins have been decreasing, and generators, operators, and consumers have faced new and uncertain regulatory and market structures for the bulk power system. As the recent situation in California has underscored, proactive measures must be taken to ensure appropriate system reliability. This work is an introduction to reliability issues for analysts in the energy efficiency area with a focus on the role of electricity demand and means for modifying demand to improve reliability. It starts with background material on the fundamentals of electricity reliability, and it then discusses factors that affect reliability, and also offers possible means to improve reliability. This study was funded by the Office of Building Research and Standards and, therefore, its focus tends to be on the role of efficiency standards in reliability.