Musicians and non-musicians were presented with short musical phrases that were either selected from the classical musical repertoire or composed for the experiment. The phrases terminated either in a congruous or a 'harmonically', 'melodically', or 'rhythmically' incongruous note. The brain waves produced by the end-notes differed greatly between musicians and non-musicians, and as a function of the subject's familiarity with the melodies and the type of incongruity. The timing of these brain waves revealed that musicians are faster than non-musicians in detecting incongruities. This study provides further neurophysiological evidence concerning the mechanisms underlying music perception and the differences between musical and linguistic processing.