The amplitude of bipolar electrograms (EGMs) is directionally sensitive, decreasing when measured from electrode pairs oriented oblique to a propagating wavefront.Use computational modeling and clinical data to establish the mechanism and magnitude of directional sensitivity.Simulated EGMs were created using a computational model with electrode pairs rotated relative to a passing wavefront. A clinical database of 18,740 EGMs with varying electrode separation and orientations was recorded from the left atrium of 10 atrial fibrillation patients during pacing. For each EGM, the angle of incidence between the electrodes and the wavefront was measured using local conduction velocity (CV) mapping.l A theoretical model was derived describing the effect of changing angle of incidence, electrode spacing, and CV on the local activation time (LAT) difference between a pair of electrodes. Model predictions were validated using simulated and clinical EGMs. Bipolar amplitude measured by an electrode pair is decreased (directionally sensitive) at angles of incidence resulting in LAT differences shorter than unipolar downstroke duration. Directional sensitivity increases with closer electrode spacing, faster CV, and longer unipolar EGM duration. For narrowly spaced electrode pairs (<5mm) it is predicted at all orientations.Directional sensitivity occurs because bipolar amplitude is reduced when component unipolar EGMs overlap, such that neither electrode is "indifferent." At the electrode spacing of clinical catheters, this is predicted to occur regardless of catheter orientation. This suggests that bipolar directional sensitivity can be lessened but not overcome by recently introduced catheters with additional, rotated electrode pairs.