Although some critics have recognized the theme of the past in The Glass Menagerie by Tennessee Williams, no one has yet systematically studied the importance of the theme of the past of the play. Thus, this paper intends to systematically explore the importance of the past of the play with careful textual analyses. As a "memory play," the past plays an indispensable role in multiple dimensions, and this paper will study these dimensions in the following perspectives: for all the characters of the play, the past is still haunting; the past reveals itself as a trap, as it forces Tom Wingfield to carry a heavy burden of guilt in/on the back of his conscience because of his desertion of his family. For both Amanda and Laura, the past becomes not only a deceptive nostalgia, but also a refurbished myth in which, time freezes, and fantasy creates its own reality. For Jim O'Connor, the past not only blinds his vision for any historical and social lessons he should have learnt, but also generates illusions rather than hopeful promises for him. With his own shallow past, Jim also manufactures illusions for Laura. Throughout the play, the past establishes its own ironic patterns, especially in the case of the father whose photograph is not only an ironic reminder of the past, but also a haunting influence on every member of the Wingfield family. In all cases, the past, appearing in whatever forms, is a negative force rather than a positive one, as it has either made one inescapably guilty, or beguilingly enchanted and deceived some of the characters, or ironically blinded and tricked others, or seriously cheated and crippled the innocent. Although the past is mysteriously enchanting and hauntingly nostalgic, it is indeed ironically unforgiving, misleading, deceiving, and destructive.