The thesis examines the factors that need to be considered in order to undertake successful design of user interfaces for older users. The literature on aging is surveyed for age related changes that are of relevance to interface design. The findings from the literature review are extended and placed in a human context using observational studies of older people and their supporters as these older people attempted to learn about and use computers. These findings are then applied in three case studies of interface design and product development for older users. These case studies are reported and examined in depth. For each case study results are presented on the acceptance of the final product by older people. These results show that, for each case study, the interfaces used led to products that the older people evaluating them rated as unusually suitable to their needs as older users. The relationship between the case studies and the overall research aims is then examined in a discussion of the research methodology. In the case studies there is an evolving approach used in developing the interface designs. This approach includes intensive contribution by older people to the shaping of the interface design. This approach is analyzed and is presented as an approach to designing user interfaces for older people. It was found that a number of non-standard techniques were useful in order to maximize the benefit from the involvement of the older contributors and to ensure their ethical treatment. These techniques and the rationale behind them are described. Finally the interface design approach that emerged has strong links to the approach used by the UTOPIA team based at the university of Dundee. The extent to which the thesis provides support for the UTOPIA approach is discussed.