This paper aims to take a step towards overcoming two lacunae in Professional Military Education (PME) literature. First, the lack of conceptualisation of what PME is and its relationship to military professionalism. Second, the fact that the vast majority of PME literature consists of studies concerning a single PME organisation, e.g. a specific military academy or college; there is almost a complete lack of national, regional and global studies of PME. The current paper takes a first step in overcoming the first lacuna by defining PME as an institution and taking a historical institutionalist approach to tracing the evolution of PME in the Western world. This new approach will also demonstrate how a sound conceptualisation of PME could contribute to overcoming the second lacuna.