This paper presents empirical evidence on the effect 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 2,764 manufacturing firms, taken from the Spanish Survey of Technological Innovation 2000. Our results suggest that, rather surprisingly, with a high level of internal technological capabilities derived from in-house R&D activities, external knowledge acquisition from scientific agents loses its importance as a determinant of firm innovation output.