This is the first collective study of the thinking behind the effective altruism movement. This movement comprises a growing global community of people who organise significant parts of their lives around the two key concepts represented in its name. Altruism is the idea that if we use a significant portion of the resources in our possession--whether money, time, or talents--with a view to helping others then we can improve the world considerably. When we do put such resources to altruistic use, it is crucial to focus on how much good this or that intervention is reasonably expected to do per unit of resource expended (as a gauge of effectiveness). We can try to rank various possible actions against each other to establish which will do the most good with the resources expended. Thus we could aim to rank various possible kinds of action to alleviate poverty against one another, or against actions aimed at very different types of outcome, focused perhaps on animal welfare or future generations. The scale and organisation of the effective altruism movement encourage careful dialogue on questions that have perhaps long been there, throwing them into new and sharper relief, and giving rise to previously unnoticed questions. In this volume a team of internationally recognised philosophers, economists, and political theorists present refined and in-depth explorations of issues that arise once one takes seriously the twin ideas of altruistic commitment and effectiveness.