This paper presents and assesses a participatory research process that employed the Delphi technique to co-produce a sustainability assessment toolkit for upland estate management in Scotland. Upland areas in Scotland have a distinct pattern of ownership, with much of the land divided into 'estates' that are owned predominantly by a range of private individuals and organisations, as well as public agencies and non-governmental and community organisations. Management objectives and land uses are wide-ranging and attract the interest of many different stakeholders. As a result, translating sustainability principles for generic, practical application in this context is a complex challenge that has not yet been addressed in an inclusive and participatory manner. In response, a reflective research approach engaged an anonymous, mixed group of academic and non-academic stakeholders over a four-stage, interactive Delphi process which lasted 18 months. The process produced a toolkit within which five principles for sustainable upland estate management and 12 corresponding 'actions' were translated into operational practice. The toolkit provided a sustainability framework for estate owners, managers and other stakeholders to assess progress towards delivering a series of practical actions on individual estates. The paper explores the implications of this toolkit for upland estate management in Scotland and reflects on the benefits and wider application of this type of research approach.