Foreign Direct Investment (FDI) acquired an important role in the international economy after the Second World War. Theoretical studies on FDI have led to a better understanding of the economic mechanism and the behavior of economic agents, both at micro and macro level allowing the opening of new areas of study in economic theory. To understand foreign direct investment must first understand the basic motivations that cause a firm to invest abroad rather than export or outsource production to national firms. The purpose of this study is to identify the main trends in FDI theory and highlight how these theories were developed, the motivations that led to the need for new approaches to enrich economic theory of FDI. Although several researchers have tried to explain the phenomenon of FDI, we cannot say there is a generally accepted theory, every new evidence adding some new elements and criticism to the previous ones.