We investigate how multinational firms contribute to the transmission of shocks across countries using a large firm-level dataset that contains ownership information for 8 million firms in 34 countries. We use these data to document two novel empirical patterns. First, foreign affiliate and headquarter sales exhibit strong positive comovements. Second, shocks to the source country account for a significant fraction of the variation in sales growth at the source-destination level. We propose a parsimonious quantitative model to interpret these findings and to evaluate the role of multinational firms for international business cycle transmission. For the typical country, the impact of foreign shocks transmitted by all foreign multinationals combined is non-negligible, accounting for about 10% of aggregate productivity shocks. On the other hand, since bilateral multinational production shares are small, interdependence between most individual country pairs is minimal. Our results do reveal substantial heterogeneity in the strength of this mechanism, with the most integrated countries significantly more affected by foreign shocks.