This Technical Report covers a) the results of a study into, and comparison between, existing classification and categorization schemes for reference materials, b) the development of reference material (RM) features and characteristics upon which a harmonized and consistent categorization scheme could be based, and c) approaches for making the categorization scheme adaptive to new RM needs and developments. The development of a harmonized categorization scheme aims at facilitating a transparent and comparable presentation of reference materials by producers, and the identification of reference materials by users. The intended categorization scheme was conceived to meet the needs of modern forms of information presentation and retrieval, i.e. internet-based catalogues and databases, and has been developed specifically with the view to being used this way.