Effective altruism is a social movement that aims to improve the way we help others. By combining reflections on the ethics of giving with a more updated understanding and data of what is an effective choice, its proponents seek to ameliorate the impact that each person can have on the welfare of others, in particular the worst-off. It has been criticized for focusing on the consequences of disadvantage, thus overlooking its deep causes. In this paper I provide a conceptual analysis of an underexplored issue in the debate about effective altruism: its theory of effectiveness. First, I distinguish effectiveness from efficiency and claim that effective altruism understands effectiveness through the lens of efficiency. Then, I discuss the limitations of this approach in particular with respect to the charge that it is incapable of supporting structural change. Finally, I propose an expansion of the notion of effectiveness of effective altruism by referring to the debate in political philosophy about realism and the practical challenge of normative theories. I argue that effective altruism, both as a social movement and as a conceptual paradigm, would benefit from clarifying its ideal, taking into account the role of institutions, and expanding its idea of feasibility.