This chapter discusses the specific contribution that the two concepts can make to unemployment and poverty research, but also important ambiguities which threaten their usefulness. It considers the theoretical framework of the terms ‘social exclusion’ and ‘underclass’, and discusses major questions which they address, and shows how they can guide empirical research. In the international discussion, the notions ‘exclusion’, ‘social exclusion’, and ‘underclass’ are theoretically and empirically highly controversial. While comparative studies of ‘social exclusion’ have become more numerous on the international level, there is a considerable lack of research dedicated to the comparison of different forms of exclusion within national contexts. The forms which exclusion takes on in the different dimensions, and the relative importance of each dimension in the entire picture of social exclusion, vary from country to country. ‘Inclusion’ extends into different dimensions of social life; it implies economic, cultural, social, and political participation.