What duties do the global affluent have towards the global poor? This book offers a philosophical inquiry into the effective altruist's answer. According to effective altruism, the global affluent have a duty to assist the global poor cost-effectively. I argue that there are two ways in which effective altruism fails to account for the moral imperative of Do No Harm. First, it fails to account for our negative duties. Second, it neglects the impermissibility of harm that is brought about as a means to an end. To align better with our intuitions, and ultimately become more impactful, effective altruism should incorporate sensitivity to these types of harms. With the principle of cost-effectiveness becoming increasingly influential in donor decision-making this book offers a timely perspective.