The Covid pandemic led us to ask how the behavior of an individual depends on its interactions with others, and how its actions then change the rest of the system: each person’s decision to wear a mask, get vaccinated, or go to a crowded bar, influences the course of the pandemic, not just that person’s own chances of infection. Collective behavior of this sort manages to operate without central control, using only interactions of individual participants. It is widespread in nature, from brains to ant colonies. This seminar examines the question, so fundamental throughout biology: how do interactions among individuals — genes, neurons, ants, elephants — generate collective behavior?