This article discusses transnational professional knowledge in action and puts forward a conception of how professionals play off different forms of knowledge to provide policy solutions and, in doing so, generate markets for their services. The key concept is ‘epistemic arbitrage’—when professionals exploit opportunities between bodies of professional knowledge. Professionals mediate between knowledge pools for strategic advantage and, if successful, they can become the ‘arbiters’ on what knowledge and practices are most influential in their area of transnational governance. Organizations also have a demand for epistemic arbitrage, as it permits managers to have more influence over what knowledge is dominant and allows greater control of staff. The concept of epistemic arbitrage is situated in the relevant literature from International Relations, Organization Studies, and Sociology, and this interdisciplinary article brings them together to advance the new transnational sociology of professions. I suggest that transnationality matters for how professionals behave and one can draw a distinction between ‘thin’ transnational environments that are especially permissive of epistemic arbitrage and professional mobilization, and ‘thick’ domestic environments where professional jurisdictions can dampen epistemic arbitrage. The article outlines how transnational professionals can be understood as unique groups that have careers and operate in networks where there is a supply of, and demand for, epistemic arbitrage. It also establishes why ‘knowing well’ is crucial to those who have power in shaping how transnational issues are governed.