Trisectorial models of economic functioning have been proposed to replace the dualistic models that proved incapable of illuminating postwar employment trends in developing countries. The new models propose 3 sectors: the subsistence sector, where average productivity corresponds to the subsistence minimum and which is thus incapable of generating a surplus for savings; the intermediate sector, weakly capitalistic, characterized by the absence of a permanent salaried work force or codified labor relations, in which precariousness of employment and the exploitation of specific social relations allow a low wage rate, with a concommitant mode of regulation that largely escapes state control; and the intensely capitalistic sector, with a salaried work force, codified labor relations, existence of administered prices, various state subventions and protections and a monopolistic type of regulation. The 3 sectors are described in greater detail and represented graphically, along with a critique of the limitations of most studies employing a trisectorial perspective. A study of the impact of demographic pressure at different levels of technology embedded in specific sociohistoric systems follows. The final section contains an analysis of 3 types of effects which may mediate the role of demographic pressure in the choice of technologies: effects of demographic pressure on structures of production and consumption, on segments of the labor force, and on involutive and evolutive processes. It is argued that the links between demographic pressure, technologic choices, and the productive sector can only be analyzed in specific social systems.