and Key Results■ After thirty years of research on the relationship between internationalization and firm performance, findings on direction and magnitude are still contradictory. This paper quantitatively reviews prior research in an attempt to reconcile the fragmented results.■ We address the question if and how internationalization relates to firm performance by integrating findings from 36 studies (41 samples, N = 7,792) using the method of meta-analysis.■ We find empirical support for a significant positive relationship at the aggregate level. Equally important, meta-analysis reveals that the relationship is moderated by R&D intensity, product diversification, country of origin, and firm age and size.