This thesis presents the development of a new and separate market for trading Demand Response (DR) in a deregulated power system. This market is termed Demand Response eXchange (DRX), in which DR in the form of hourly load reduction is considered a product to be negotiated between two groups of market participants, namely buyers and sellers. DR buyers, including all transmission companies (Transcos), distribution companies (Discos), and retail companies (Recos) need DR for their risk management benefits related to, for instance, transmission and distribution network security, and electricity market volatility. Sellers, including all Energy Service Companies (ESCos), are capable of significantly modifying electricity customers demand to supply DR on request. The trading between these sellers and buyers is settled by a new system operator termed DRX operator (DRXO). Two alternative market clearing schemes, namely pool-based and agent-based, are developed as the ground technical mechanism of a DRX. In the former scheme, all sellers and buyers are required to submit offers and bids reflecting their marginal costs and benefits, respectively, derived from a set of DR quantities. Based on this collected information, the DRXO will clear the market by centrally maximizing the total benefit for all participants under some economic constraints, i.e., demand-supply balance. In the other hand, in an agent-based scheme the participants are viewed as economic agents that behaves in a self-interested manner. The scheme will be designed to constrain each agent to ensure optimal global efficiency, while also allowing agents to maximize their own profits locally. In order to evaluate these DRX schemes, we develop a comprehensive assessment framework using certain economic valuation methodologies such as cost-benefit analysis and externalities treatment. Firstly, DR cost and benefit for each pariticipant in the market (i.e., buyers and sellers) are analysed in detail. Based on this local analysis, a global evaluation is performed to determine whether the optimized DR can give a positive social surplus. If so, the DR will be dispatched during the hour under consideration. Using this newly developed assessment framework, we demonstrate the advantages of DRX over conventional DR trading/scheduling schemes. Throughout the thesis, both analytical proofs and numerical examples are provided to substantiate the advantages of the proposed DRX schemes. Our formulations rely on a wide range of theories: demand{supply modeling with a competitive market equilibrium, cost-benefit analysis, spot pricing of electricity, and network reliability assessment. Numerical simulations are performed on various test systems, including the Roy Billinton Test System (RBTS), to illustrate the effectiveness of DRX in analysing and optimizating DR benefits.