School middle leadership is a complex and increasingly important school leadership position, with research showing the significance of middle leadership to school improvement and teacher development. The purpose of this article is to identify and analyse empirical peer reviewed articles on middle leadership from 2006 to 2020, to understand how middle leaders are defined, the responsibilities they hold, the impact and professional development they are afforded, and to discover lacunae in studies to support further research. The review was conducted using Scopus and ERIC; a total of 35 articles across 14 countries were selected. Using Nvivo, narrative synthesis was utilised to analyse the articles resulting in four conclusions: (1) middle leadership is difficult to define; (2) middle leadership positions and responsibilities vary considerably and are best understood in context; (3) middle leaders directly and indirectly impact teacher practice, team development, school reform and professional learning, although there is limited direct research into their impact; and (4) middle leadership professional learning has not progressed to the point to adequately equip middle leaders for the complexity of their positions. The article highlights two distinguishable differences to separate teachers and middle leaders, and provides an operational definition, elaborated description and set of middle leadership characteristics to guide future research and policy advancement.