This paper provides compelling evidence that large-scale market failures and barriers prevent consumers in the United States from obtaining energy services at least cost. Assessments of numerous energy policies and programs suggest that public interventions can overcome many of these market obstacles. By articulating these barriers and reviewing the literature on ways of addressing them, this paper provides a strong justification for the policy portfolios that define the "Scenarios for a Clean Energy Future," a study conducted by five National Laboratories. These scenarios are described in other papers published in this special issue of Energy Policy.