This paper reviews the literature on two-sided atomeless assignment models of workers to tasks. Using simple parametric examples, the fundamental differences between the comparative advantage and the scale of operations models are illustrated. Holding the distributions of abilities and tasks and the production function of worker-task pairs constant, the two principles are shown to produce different wage distributions and wage inequality. These models are useful to evaluate the general equilibrium effect of technical change on the wage structure. In all models, skilled-biased technical change that impacts the production function of worker-task pairs lead to rising wage inequality.