This thesis seeks to offer a substantial contribution to a new understanding of brochs. It reviews the subject of brochs in their main area of occurrence in Atlantic Scotland with the objectives of improving the information base; of recasting the conceptual basis of broch study; and of drawing up a strategy for further research and selective excavation. The results of original research in Caithness, Sutherland, and Skye, are presented, comparing and contrastingjthese with the results of research work by others in Orkney and Shetland. The original research is based on a detailed field analysis, including new survey plans and descriptions, of individual sites within selected study areas; a reassessment of old excavations; and a consideration of the contemporary environment of brochs. The thesis concludes by considering the implications of the research results for the traditional understanding of the nature of a broch, pointing out that brochs are generally dissimilar from their popular textbook image; by examining difficulties of terminology and classification arising from the new understanding; by discussing approaches to broch study; and by suggesting an amended conceptual basis for further study of brochs.