Although it has its roots in the work of Kerr (1954), on balkanisation of labour, and other US institutionalists,' segmented labour market (SLM) theory, at least by that name, is generally acknowledged to be little more than a quarter of a century old. Even so, it already has a history, divided into distinct periods. It began with the dual labour market (DLM) theory of the early 1970s, and this gave way to a more general approach, SLM theory itself, both in terms of number of labour market segments and range of explanatory factors. From the outset, the theory was heterodox, certainly as far as neoclassical economics was concerned, although SLM theory has found more favour within other social sciences in which social structures, institutions and relations are conventionally to be found. By the mid-1980s, neoclassical economics began to abandon its hostility to SLM theory and to contribute a distinctive analysis of its own which has developed alongside and influenced the continuing radical tradition. These most recent developments are covered in the following chapter. This chapter is concerned with the first, radical phase of SLM theory.