In a nutshell, what is effective altruism and how does it differ from ordinary charitable giving? Effective altruism is both an emerging movement and the set of ideas behind that movement. The basic idea is that to live a fully ethical life, we should seek to do the most good we can. To discover what will do the most good, we need to use reason and evidence. In contrast, two-thirds of donors to charity do no research at all into the organizations to which they donate—they are moved by images that play on their emotions, but give no indication whether the organization is effective at what it claims to be doing. Aren’t we all, at the core, self-interested? The book introduces readers to many of the men and women who are practicing effective altruism. What they are doing will startle many readers—choosing their careers so that they can donate more, and donating half their incomes to effective charities. Yet typically they don’t think of themselves as making a sacrifice. They find their lives more rewarding than they were before they made these choices. So it may not be a question of denying self-interest, but of a different understanding of what really is in one’s own interests. Can effective altruism change the world? I find the stories I tell in this book immensely encouraging. There are not many effective altruists yet, but they are already changing the world, and their impact is growing.