In studies of innovation, policy entrepreneurs recognize latent demand for new policies and then expend resources to promote them. But studies of policy entrepreneurs have generally focused on the demand for innovation, while neglecting the supply side of policy entrepreneurship. This article argues that bureaucratic labor markets affect the emergence of policy entrepreneurs, and so affect the diffusion of policy innovations across local governments in the United States. Analysis of a survey of municipal police chiefs and water utility managers relates governments' hiring and promotion policies to their adoption of professionally fashionable innovations. Agency heads who advanced to their current positions diagonally (arriving from another organization) are more likely to initiate these innovations than are agency heads who were promoted from within. Bureaucratic policy entrepreneurs emerge where government demand for innovation meets a supply of mobile administrators, who carry the priorities of their professions into the agencies that they serve.