Human beings choose their friends, and often their neighbors, and co-workers, and we inherit our relatives; and each of the people to whom we are connected also does the same, such that, in the end, we humans assemble ourselves into face-to-face social networks with particular structures. Why do we do this? And how might an understanding of human social network structure and function be used to intervene in the world to make it better? Here, I review recent research from our lab describing several classes of interventions involving both offline and online networks that can help make the world better, including: (1) interventions that rewire the connections between people, and (2) interventions that manipulate social contagion, facilitating the flow of desirable properties within groups. I will illustrate what can be done using a variety of experiments in settings as diverse as fostering cooperation in networked groups online, to fostering health behavior change in developing world villages, to facilitating the diffusion of innovation or coordination in groups. I will also focus on our recent experiments with “heterogenous systems” involving both humans and “dumb AI" bots, interacting in small groups. By taking account of people's structural embeddedness in social networks, and by understanding social influence, it is possible to intervene in social systems to enhance desirable population-level properties as diverse as health, wealth, cooperation, coordination, and learning.