In recent times middle leaders have become increasingly important to the success of schools in many countries, including Australia. Correspondingly the volume of research into middle leadership in schools has increased since the late 1990s. Compared to senior leaders such as principals and headmasters, however, middle leaders remain under-researched and middle leadership remains under-theorised. Given the increasing responsibilities bestowed upon middle leaders in schools, there is a need to better understand the nature of middle leadership as well as to suggest agendas for future research. This paper is the result of an analysis of Australian and international literature that aimed to identify key themes emerging from middle leadership research dating from the 1990s to the present. A search process using multiple databases and specific selection criteria identified over 250 research articles, institutional reports, book chapters and books which were then content-analysed. Three main theme categories emerged from the analyses relating to middle leader roles, factors that influence the work of middle leaders, and ways middle leaders might influence school outcomes. Theme categories and sub-categories are described in terms of their prominence as units of investigation and inter-relationships among them are explored. Synthesis of findings from the literature led to the development of a model depicting the state of current scholarship relating to middle leadership. Implications of the model and recommendations for future research agendas around middle leadership include the need to address substantial knowledge gaps about the nature of middle leadership roles, as well as the potential impacts of the work of middle leaders on teacher performance and student outcomes.