Decommissioning of both commercial and R&D nuclear facilities is expected to increase significantly in the coming years, and the largest of such industrial decommissioning projects could command considerable budgets. Several approaches are currently being used for decommissioning cost estimations, with an international culture developing in the field. The present cost estimation practice guide was prepared in order to offer international actors specific guidance in preparing quality cost and schedule estimates to support detailed budgeting for the preparation of decommissioning plans, for the securing of funds and for decommissioning implementation. This guide is based on current practices and standards in a number of NEA member countries and aims to help consolidate the practice and process of decommissioning cost estimation so as to make it more widely understood. It offers a useful reference for the practitioner and for training programmes.