In recent years a growing amount of attention has been focused on the need to develop a cost-effective portfolio of electricity supply options that provides society with a measure of protection from such factors as fuel price volatility and supply interruptions. A number of strategies, including portfolio theory, real options theory, and different measures of diversity have been suggested. In this paper we begin by first considering how we might characterize an optimal portfolio of supply options and identify a number of constraints that must be satisfied as part of the optimization process. We then review the strengths and limitations of each approach listed above. The results of our review lead us to conclude that, of the strategies we consider, using the concept of diversity to assess the viability of an electricity supply portfolio is most appropriate. We then provide an example of how a particular measure of diversity, the Shannon-Weiner Index, can be used to assess the diversity of the electricity supply portfolio in the state of Illinois, the region served by the Midwest Independent System Operator (MISO), and the continental United States. (C) 2009 Elsevier Ltd. All rights reserved.