The paper is an empirical investigation of key internal and external sources of innovation capability in small and medium firms (SME) in the UK. An experimental measure of innovation capability is designed, which captures not merely the occurrence of innovations but also their scientific complexity and originality. The results obtained with this measure compare favourably to those obtained with more conventional statistics. A range of factors internal to firms are found to be relevant, including owners' technical education and prior working experience in large firms and R&D institutions, technical skills of the workforce, and investments in R&D and training. Significant external factors are: public financial support for R&D, and interaction with nearby R&D and training institutions. Although interaction with customers, suppliers and similar-oriented firms are more frequent than the former, there is no evidence that intensive linkages of this kind would be important for innovative capability. These findings do not support the thrust of current UK policy, which seeks to promote SME innovative performance through the formation of geographical clusters of firms in similar lines of business