Most of what we know about how ideas spread comes from a world in which "influencers" are king, "sticky" ideas "go viral," and good behavior is "nudged" forward. The problem, Damon Centola suggests, is that our beliefs and behaviors stay the same. Drawing on deep-yet-accessible research, Centola surveys the spread of coronavirus, the success of the Black Lives Matter movement, the failure of Google+, and the rise of political polarization, while offering guidelines on why 21st-century social networks often fail and how it is nonetheless possible to change the world around us.