Through its three novels, set in 1888, 1903 and 1918, Broch's Schlafwandler trilogy traces a progressive fragmentation of social values in late modernity. This article investigates a key marker of this fragmentation: the figuration of individual and collective memory, which undergoes a radical shift between Part I and Part III. In Part I the depiction of memory engages the reader with the protagonist's psychological and moral conflicts and the formation of his individual identity. In Part II memory features as abstract and collective, in allegorical meditations on man's existence in time; in Part III the theme of remembering is largely displaced by that of amnesia, emphasising the isolation of the individual in the era of 'Wertzerfall'. This depiction of cultural disintegration is, however, counterbalanced by the symbolic unity of Die Schlafwandler, whose aesthetic structures play an essential part in what Broch saw as the novel's 'cognitive' task. Here memory features within the reading process itself. To conclude we examine some of the trilogy's densely intersecting leitmotifs that activate the reader's memory in defiance of disintegration and amnesia, and thereby contribute a vital element to the realisation of the 'cognitive novel'.