The article proposes a conceptual framework for exploring processes of expert knowledge creation in policy fields using recent advances in the sociological field theory. It rests on an observation that policy fields are no longer autonomous, but increasingly embedded in public life, and include actors from other fields who create knowledge for policy-making. While existing research clarifies many aspects of the relationship between expertise and policy-making, little is known about how multiple interrelated actors create expert knowledge and how this knowledge is institutionalized in policy fields. Addressing this gap, the article outlines mechanisms of expert knowledge creation in policy fields. It argues that actors mediating between different field domains succeed in creating influential expert knowledge, as they combine knowledge from disconnected areas and influence the operation of field-level mechanisms. Drawing on examples from the literature, the authors demonstrate the application of the theoretical framework to examine difficult-to-observe knowledge creation mechanisms operating in policy fields.