The empirical relationship between educational attainment and credentials with socioeconomic attainment is well established, but why this relationship arises remains in doubt. The author of this article discusses seven types of middle-range theories meant to explain the relationship: human capital, screening (including filtering), signaling, control, cultural capital, institutional, and credentialist theories. In each, the central causal mechanism concerns how employers and job seekers acquire and use labor market information. The author argues that occupational status attainment and wage determination models are not adequate to explain the mechanisms underlying the process whereby the highly schooled become the highly placed in job hierarchies. He indicates the implications of transformations of the American labor market for further assessment of the relationship between educational credentials and job assignment.