Despite the growing importance of volunteers to many organizations, there has been little research into how they share their knowledge and develop the skills they need. Moreover, the focus of existing research has been on short-term episodic volunteering. This article addresses these deficits by examining the significance of knowledge sharing to volunteer development and also the relative importance of individual processes. A case study approach is adopted, which focuses on a major heritage site that is heavily reliant on a volunteer workforce. Development of volunteers is particularly relevant to the heritage sector where organizations need a sizeable, stable, and well-trained volunteer workforce. We performed semi-structured interviews with six managers as well as conducting five focus groups with volunteers from diverse work areas in the National Trust. The Volunteer Development through Knowledge Sharing Model is proposed. This is the first conceptual model to summarize the processes in volunteer development. Processes are clustered as follows: informal learning, formal training, learning resources, and research by volunteers. Informal learning through