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The excitation and transfer mechanism of brake judder was investigated in real operating conditions on the road. Monitoring brake parameters, especially the dynamic brake-disc geometry, during judder led to a profound understanding of the excitation mechanism. This dynamic brake-disc geometry changes virtually due to acting forces on wheel, hub and brake disc. This was found to add to brake-torque variations and thus judder. The simultaneous measurement of accelerations along the transfer path from excitation to the driver's contact points and the development of a suitable method to analyse global and local system behaviour enabled a better understanding of the transfer mechanism. A comparison of the global transfer functions of three vehicles with similar front suspensions showed highly different judder sensitivities. An analysis of the causal dependencies was carried out using local transfer functions.