This paper offers an exploration of the socialist principle “From each according to their abilities, to each according to their needs.” The Abilities/Needs Principle is arguably the ethical heart of socialism but, surprisingly, has received almost no attention by political philosophers. I propose an interpretation of the principle and argue that it involves appealing ideas of solidarity, fair reciprocity, recognition of individual differences, and meaningful work. The paper proceeds as follows. First, I analyze Marx’s formulation of the Abilities/Needs Principle. Second, (...) I identify the principle’s initial plausibility, but show that it faces serious problems that cannot be addressed without developing a fresh interpretation of it. Third, I provide an interpretation of the principle that highlights demands concerning opportunities for self-realization in work, positive duties of solidarity, sensitivity to individual differences, and mechanisms of fair reciprocity. Fourth, I discuss a possible institutional implementation of the Abilities/Needs Principle. Finally, I identify some normative puzzles about the transition from capitalism to socialism, and suggest how the Abilities/Needs Principle might gain motivational traction by mobilizing the powerful idea of human dignity. (shrink)