This article presents empirical evidence on the effects of external knowledge-sourcing strategies on the development of both product and process innovations, and assesses the degree to which such effects are influenced by the firm's internal technological capacities. In our analysis, we consider two strategies for acquiring external knowledge (buying and cooperating) and two types of external sources (industrial agents and scientific agents). The analysis is based on a sample of 1329 manufacturing firms active in innovation activities taken from the Spanish Survey of Technological Innovation 2004. We find that the effects of the knowledge-sourcing strategies differ significantly across innovation types (product or process innovation). In addition, our results indicate that although internal R&D activities are associated with a greater use of external scientific knowledge sources (through cooperation), they do not seem to promote their exploitation for innovation development, that is, to say, they do not have synergistic effects. Copyright 2009 The Author 2009. Published by Oxford University Press on behalf of Associazione ICC. All rights reserved., Oxford University Press.