Relying upon original empirical evidence gathered in 12 European metropolitan areas, this paper focuses on conditions characterizing the generation of localized technological knowledge. Complementarities relying upon favorable industrial and institutional conditions support collective learning, which in turn emerges as the determinant in the generation of new knowledge. Technological knowledge emerges as a collective good both from a structural and a dynamic viewpoint. Knowledge production is the result of a process that relies upon diverse and yet interdependent knowledge bases, which afe systematically accessed, accumulated and recombined through different interpolating processes. Implications for regional and local innovation systems are thus raised appreciating connectivity between the variety of knowledge producers, and therefore communication opportunities and communication channels as central concerns far knowledge-enhancing technology policy.