Accurately defining the full extent, scope, and reach of what it means to have professional expertise in sport coaching is difficult in the absence of true professional bodies, and the wide ecosystem of sport within which it operates. Whilst there are numerous models, criteria, and key principles attached to the idea of expertise in sport coaching, it is the contention of this opinion piece that – given the widely held appreciation that sport coaching requires knowledge of how to interact with others – the concept of reflexivity can be particularly useful in determining the extent to which coaches can demonstrate, and effect, expertise. The context and rationale for this, and subsequent implications, are discussed within the paper.