The unbalanced growth model is based around exploiting scale economies and complementarities in favoured sectors that can induce more investment and productivity growth. Those sectors, in Hirschman’s framework, have more backward and forward linkages; the former referring to provision of inputs from other activities and sectors, the latter to demand for new activities. The development policy challenge is, accordingly, about identifying lead sectors, addressing missing linkages and strengthening inter-industry and intersectoral interdependencies to boost productivity growth. In this sequence, infrastructure would follow rather than lead the growth process. It is laigely around this sequencing issue that differences between balanced and unbalanced growth strategies emerged in early development policy debates (see box 4.2). Despite these differences, there was general agreement that in most developing countries, investment in general, and in infrastructure, in particular, involves a series of non-marginal adjustments that are poorly coordinated by markets and for which planning techniques of various kinds are desirable.