System reliability is an integral part of a properly designed deregulated electricity market, even though wholesale energy prices are its most visible piece. This paper discusses the re-sources, collectively known as the ancillary services, in a power system for maintaining reliability. It then discusses an integrated approach in systematically determining and securing the needed ancillary services and system configuration across multiple time spans, culminating in the cooptimization of energy and ancillary services for security-constrained generator unit commitment in the day-ahead market and security-constrained economic dispatch of the generators in real-time operation. The paper also provides a summary of the ancillary services market in several U.S. power markets and proposes several technical issues for further discus-sion and research. Keywords—Ancillary services, deregulated electricity market, multiple settlement system, reliability, security-constrained eco-nomic dispatch (SCED), security-constrained unit commitment (SCUC). I.