Short-term interactions between people are thought to play an important role in overcoming frictions to knowledge diffusion. I estimate the effect of temporary colocation on knowledge flows using a setting where colocation is plausibly exogenous. I define a pair of scientists to be colocated if they serve together as grant reviewers on an NIH study section. Using variation in the timing of when reviewers serve on a study section, I find that short-term interactions lead to an increase in citations between pairs of scientists. The effect persists up to 10 years after the scientists’ initial meeting and increases with the number of interactions that they had. This suggests that familiarity with, and not just awareness of, another scientist is an important factor in knowledge diffusion. Slides available here.