AbstractThe Energy Efficiency First (EE1st) principle has recently been placed onto the political agenda in the European Union (EU). While the general rationale for EE1st is described in EU legislation and supporting literature, a common understanding of the principle’s implications for energy-related planning, investment, and policymaking is still missing. Based on an exploratory review of the literature, the objective of this article is to improve the theoretical understanding of EE1st. First, it develops a conceptual framework, describing EE1st as a decision-making principle that prioritises demand-side resources over supply-side alternatives whenever these provide greater value to society in meeting decision objectives. Second, it highlights the unique aspects of EE1st by systematically comparing the principle with associated concepts, such as Integrated Resource Planning. Third, it provides theoretical justification for EE1st by describing the economic rationale behind the principle. Fourth, it outlines policy considerations for its practical implementation. In sum, the EE1st principle is shown to have a compelling theoretical background that can help inform the design of effective policy interventions in order to move from principle to practice.