Teamwork in research has been on the rise and so has the size of R&D teams. This paper offers an explanation for increasing team size that we call the "racing against time" hypothesis: With innovation races more competitive globally, R&D firms need to finish research projects as quickly as possible and therefore have an incentive to put together a team with more R&D personnel. We test this hypothesis against a natural experiment that took place in 1995 when the U.S. patent law was amended.