This paper addresses the question of whether patent citations are useful indicators of technology flows. We exploit the distinction between citations added by inventors and patent examiners. We use information from the search reports of european patent office patent examiners to construct our dataset of patenting activity in europe and the us, and apply various econometric models to investigate what determines the probability that a citation is added by the inventor rather than the examiner. Contrary to previous work which uses us patent and trademark office data, we find that geographical distance is a factor that strongly diminishes the probability of knowledge flows. We find other significant effects of such factors as cognitive distance, time and strategic factors on citing behaviour.