The current distribution of power markets around intermediate structures that fall between the two extremes of full integration and unbundling suggests that there has not been a linear path to power market structure reform. Rather, many developing countries may retain intermediate structures into the foreseeable future. This possibility exposes a gap in the understanding of power market structures, since most theoretical work has focused on the two extreme possibilities and there is limited evidence of the impact of unbundling for developing countries. Power Market Structure takes a novel analytical approach to modeling market structure, together with ownership and regulation, in determining performance across several indicators, including access, operational and financial performance, and environmental sustainability. Its conclusions--which will be of particular interest to policy makers, academics, and development practitioners--reflect evidence drawn from statistical analysis and a representative sample of 20 case studies, selected based on initial conditions such as income and power system size. The key result of the analysis is that unbundling delivers results when used as an entry point to implementing broader reforms, particularly introducing a sound regulatory framework, and reducing the degree of concentration of the generation and distribution segments of the market by attracting additional public and private players and greater private sector participation. In addition, there seems to be a credible empirical basis for selecting a threshold power system size and per capita income level below which unbundling of the power supply chain is not expected to be worthwhile. Partial forms of vertical unbundling do not appear to drive improvements. The most likely reason is that the owner was able to continue exercising control over the affairs of the sector and hinder the development of competitive pressure within the power market.