As the largest non-governmental preservation association in the world, the National Trust for England, Wales and Northern Ireland (the National Trust) is arguably one of the most influential organizations of its kind. As such, it provides particularly useful case studies for examining the process of heritage construction, showing how definitions of heritage are institutionalized and modified over the course of history. This is the first book to provide a systematic and scholarly analysis of the history, development and transformation in the heritage activities of the National Trust - the world's first national heritage organization. Based on extensive research, it examines how and why definitions of 'legitimate' national heritage emerge and change over time, and who and what is central to the defining and legitimating process. Illustrated with case studies from a variety of National Trust properties, this book reveals the unique role non-profit organizations play in the processes of defining heritage and links the work of this important heritage organization to the fields of cultural sociology, organizational analysis and the interdisciplinary field of heritage studies. Using an analysis of archival documents, participant observation, interviews and case studies, this research shows that the National Trust has transformed its definition of national heritage at several key points in its history. These different stages are characterized by different types of culture preserved during the organization's 100+ year history.