This paper studies the impact of university-industry collaboration on academic research output. We report findings from a unique longitudinal dataset on all the researchers in all the engineering departments of 40 major universities in the UK for the last 20 years. We introduce a new measure of industry collaboration based on the fraction of research grants that include industry partners. Our results show that productivity increases with the intensity of industry collaboration, but only up to a certain point. Above a certain threshold, research productivity declines. Our results are robust to several econometric estimation methods, measures of research output, and for various subsamples of academics.