The aim of this thesis is to evaluate the 51st (Highland) division over the course of the First World War. Underpinning the study is an analysis of both change and continuity, at home and overseas, and the performance of the division as a fighting unit. The key themes identified for study have been training, esprit de corps, recruitment and reinforcement, and battle performance. Through the investigation of the key themes, other important characteristics have been analysed, such as command and control, organisation, and the level of centralisation in both the formation and in the wider Army. Key questions in the research apply to both divisional study and to wider academic understanding of the First World War. The thesis considers a number of themes that have been neglected by historians old and new, and brings into sharp focus some areas of research that may have produced inaccurate assumptions. In addition, a substantial range and quantity of primary sources have been utilised, many unexplored until now. The selection of the 51st (Highland) Division for study was based on a number of criteria. (Highland) Division experiences were both unique and not unique. In some areas it was a very individual formation, but in other areas or at particular times of the war it was not.