This study summarizes the main conclusions from a systematic review of the empirical literature regarding the impact on firms of the use of knowledge providers (universities, research institutes and knowledge intensive business services). With the aim to organize the literature, we classify the different works according to the research question addressed: (i) Which firms use knowledge providers?; (ii) Do firms using knowledge providers achieve better results?; (iii) Which firms benefit more from using knowledge providers? Stylized facts are that larger, more R&D intensive and high tech firms are more likely to use knowledge providers and that use of knowledge providers is associated to firms higher technical results. Less attention has been paid to the third question and no stylized facts can be developed on it. Several recommendations for future research emerge. First, to take in greater consideration methodological issues so that potential biases in the results, caused by sample selection and endogeneity, are handled properly. Second, to develop comparative analysis of the differential features of different knowledge providers. Third, to pay more attention to the determinants of impact, and fourth, to take into account depth and breadth of collaborations.