Demand response（DR） is the expansion of demand side management（DSM） in competitive electricity markets. DR is an important measure to ensure system security and economy operation by using price signals and incentive mechanisms. Based on DR researches and experiences around the world， a comprehensive research on DR in electricity markets is conducted. DR programs in electricity markets are classified into two categories， i.e. price-based DR and incentive-based DR. Then detailed light is shed on two key problems， DR implementation mechanisms and impacts of DR on power systems. Furthermore， cost-benefit of implementing DR programs is analyzed， and decision process of customers participating in DR programs is thereon discussed. Suggestions and assumptions of developing DR are finally introduced in view of China's electricity markets reform， and several important problems which need to be further studied are proposed.