International trade theory has traditionally examined protection from a normative perspective: What are its consequences and when is it justified from the point of view of national welfare? Beginning in the 1980s increasing attention has been paid to analyzing protection from a positive perspective: What actually determines it? These two approaches are obviously complementary — both are necessary. This chapter addresses the latter. Other surveys are provided by Nelson (1988), Magee (1994) and Rodrik (1995).