This paper argues that the market rules governing the operation of a re-structured electricity market in combination with its market structure can have a substantial impact on the ability of participants in the market to exercise market power. Using evidence on the design of electricity markets in England and Wales, Norway, the state of Victoria in Australia and New Zealand, this paper illustrates that market structure and market rules are important drivers of the behavior of prices in a competitive electricity market. The paper first summarizes the important features of the market structure and market rules in each country. Then it proceeds to provide an assessment of the relationship between market rules and market structure and the behavior of prices in each market. The paper closes with a short discussion of the evidence that the behavior of prices in each country is the result of the exercise of market power.