Least-cost utility planning is a new way for utilities and state regulatory commissions to assess consistently a variety of demand and supply resources that cost-effectively meet customer energy-service needs. This new planning paradigm (a) explicitly includes conservation and load-management programs as energy and capacity resources; (b) includes consideration of environmental and social factors, as well as of direct economic costs; (c) involves public participation; and (d) includes careful analysis of the uncertainties and risks posed by different resource portfolios and by external factors. The relative paucity of data on demand-side resources, particularly when compared to the information available on supply resources, poses a significant barrier to integrating these options into utility resource plans. In this study, we briefly compare the data and assumptions available on supply resources with those available on demand-side resources. We then discuss the types of data that are needed to assess existing patterns and trends in electricity use (baseline data), the costs and performance of demand-side technologies, and the effects of demand-side programs. Our analysis suggests that evaluation of program effects is the area with the greatest need for additional attention.