Over the last decade policies framing the enterprise agenda for UK higher education institutions (HEIs) have consistently emphasized the potential impact of successful universities on both regional and national economies. Such policies have been backed by significant public funding to ensure that the UK HEI sector is able to compete globally in the creation and exploitation of cutting-edge science. Collectively, these initiatives have led to the development of a "third mission" for universities--increasing the pace and impact of knowledge-exchange activities--while simultaneously maintaining and improving their more traditional research and teaching activities. Previous studies have demonstrated the contribution of universities to the growth of high-technology clusters as well as to knowledge exchange activities more generally. However, the extent to which these activities, particularly the formation of new university-related companies, deliver benefits to particular regions and cities is imperfectly understood. As a consequence, the economic and social impact of companies originating in universities in the UK is generally under-reported by official surveys, which frequently fail to capture details of the trajectories of such companies after formation. These considerations form the context for this paper, which presents the results from a detailed study of the impact of science and technology based academic spin-offs from a cross-section of London's HEIs. (Contains 10 tables, 4 figures and 12 notes.)