Early in the book, the reader learns that Winston is constantly struggling to remember his past, and that he often fails:
It was no use, he could not remember: nothing remained of his childhood except a series of brightly-lit tableaux, occurring against no background and mostly unintelligible.
But there are flashbacks throughout. One significant memory sequence occurs after the Two Minutes of Hate session where he thinks O'Brien sympathizes with him. He remembers a dream he had where a person approached him and told him that he would meet him in a "place where there is no darkness." Winston identifies this voice with O'Brien, with disastrous consequences.
Wiinston also has a poignant dream about his mother and sister, neither of whom he really remembers. They are staring at him from what seems to be an underground, or even underwater place, and he is overwhelmed by the feeling that their lives had been "sacrificed to his own." This sequence serves to show what has been lost since the rise of the totalitarian state:
The thing that now suddenly struck Winston was that his mother's death...was tragic and sorrowful in a way that was no longer possible. Tragedy...belonged to the ancient time, to a time when there was still privacy, love, and friendship...
Another flashback is used to suggest what has happened to cause the rise of Big Brother. Winston remembers his father hauling him down a flight of stairs to a Tube station which served as a bomb shelter. "Since that time," Winston remembers, "war had been literally continuous." Flashbacks are thus used to demonstrate Winston's yearning for humanity, to show what the totalitarian state has taken from people, and to provide background for the events portrayed in the novel.