Employing original, representative survey data, we document that cognitive, interpersonal and physical job task demands can be measured with high validity using standard interview techniques. Job tasks vary substantially within and between occupations, are significantly related to workers' characteristics, and are robustly predictive of wage differentials both between occupations and among workers in the same occupation. We offer a conceptual framework that makes explicit the causal links between human capital endowments, occupational assignment, job tasks, and wages. This framework motivates a Roy (1951) model of the allocation of workers to occupations. Tests of the model's implication that "returns to tasks" must negatively covary among occupations are strongly supported.