This paper derives a general relationship among intra-industry trade, specialization, and gravity. This relationship holds in any two-country trade model under assumptions that are frequently adopted in theoretical trade models. Using this general relationship, we can take an integrated approach to the theory of intra-industry trade. In two-good, two-factor models, this relationship implies that the intensity of intra-industry trade is decreasing in the difference in factor proportions and is increasing in the product of per-worker GDPs. Thus, the popular assertion that a negative correlation between the intensity of intra- industry trade and the difference in factor proportions supports monopolistic competition is groundless. Using panel data on bilateral trade flows, we estimate these relationships. We find that the negative correlation between intra-industry trade and specialization is quite robust. We also find a robust positive correlation between intra-industry trade and the product of per-capita GDPs.