This paper investigates peer effects at the level of individual connections, leveraging the approach to shed light on peer mechanisms. In a field experiment using college freshmen, we elicited best friends and offered monetary incentives for gym visits to a treated subset. We find large spillovers from treated subjects to treated best friends but none from treated subjects to control best friends. We also find evidence of a mechanism: Subjects coordinate by visiting the gym with best friends, indicating that the intervention harnesses complementarities in utility or commitment mechanisms. Results highlight subtle peer effects and mechanisms that often go undetected.