The role of expertise in policy making has been a focus of political science research in recent decades. Underlying formal models in this area is a conception of expertise that is very simple: expertise is a single piece of information. Combined with a condition on the set of possible processes, this simplicity implies that expertise is invertible. Thus, a single recommendation by an expert can render a layperson an expert. In this paper, I offer a broader representation of expertise and policy making that relaxes these features. To demonstrate that this generality matters to political behavior, I develop a simple model of delegation and show that imperfect invertibility of expertise provides a resolution of the commitment problem of legislative–bureaucratic policy making. The theory predicts that only issues of sufficient complexity can be delegated, consistent with anecdotal evidence.