This paper addresses the pattern of knowledge flows as indicated by patent citations between European regions. Our findings support the hypothesis that there are important barriers to knowledge flows in Europe. Patent citations occur more often between regions which belong to the same country and which are in geographical proximity. Furthermore, patent citations are industry specific and occur most often between regions that are specialised in industrial sectors with specific technological linkages between them. Patent citations are also more frequent when the citing region belongs to the same linguistic group as the cited region.