Several recent papers have used plant-level data and panel econometric techniques to carefully explore the existence FDI externalities. One conclusion that emerges from this literature is that it is difficult to find evidence of positive externalities from multinationals to local firms in the same sector (horizontal externalities). In fact, many studies find evidence of negative horizontal externalities arising from multinational activity while confirming the existence of positive externalities from multinationals to local firms in upstream industries (vertical externalities). In this paper we explore the channels through which these positive and negative externalities may be materializing, focusing on the role of backward linkages. In particular, we criticize the common usage of the domestic sourcing coefficient as an indicator of a firm's linkage potential and propose an alternative, theoretically derived indicator. We then use plant-level data from several Latin American countries to compare multinationals' linkage potential to that of domestic firms. We find that multinational's linkage potential in Brazil, Chile and Venezuela is higher than for domestic firms. For Mexico, we cannot reject the hypothesis that foreign and local firms have similar linkage potential. Finally, we discuss the relationship between this finding and the conclusions that emerge from the recent empirical literature.