The cofounder of the Effective Altruism movement presents a counterintuitive approach anyone can use to make a difference in the world. While studying philosophy at Oxford University and trying to work out how he could have the greatest impact, William MacAskill discovered that most of the time and money aimed at making the world a better place achieves little. Why? Because individuals rarely have enough information to make the best choices. Confronting this problem head-on, MacAskill developed the concept of effective altruism, a scientific, data-driven approach to making a difference that operates by asking these five questions: How much will this action improve others' lives? Is this the most effective action I can take? How useful is my contribution, given what others are already doing? What will happen if I don't do it? What are the chances of success, and how good would success be? Applying these principles, he demonstrates that many of our assumptions about doing good are misguided: he argues that one can potentially save more lives by working on Wall Street than as a heart surgeon, that cancer charities are not the best use of money, and that buying sweatshop-produced goods is a form of ethical consumption. MacAskill challenges us to think differently but argues that if we set aside our biases and rely on evidence and careful reasoning, each of us can make a tremendous difference.