Shopping momentum occurs when an initial purchase provides a psychological impulse that enhances the purchase of a second, unrelated product. The authors propose that the most promising theoretical mechanism for shopping momentum comes from Gollwitzer's (1990) theory of implementation and deliberation mind-sets. Under this theory, shopping momentum occurs because the initial purchase moves the consumer from a deliberative to an implementai mind-set, thus driving subsequent purchases. After demonstrating the main shopping momentum effect, the authors support the mind-set theory by (1) demonstrating how an initial purchase induces implementai orientation and (2) by illustrating that an implementation mind-set leads to greater purchase. The authors then explore the boundaries of this effect by demonstrating how shopping momentum can be interrupted. Finally, they discuss alternative theoretical accounts for the results and explore consequences for marketing managers. © 2007, American Marketing Association.