Packaging influences usage behavior long after it has influenced purchase. Managers of consumer packaged goods and public policy officials have, therefore, questioned whether a package's size influences usage volume. Although often assumed, it has never been supported. Four laboratory studies and a final study in a Laundromat identify circumstances in which larger package sizes encourage greater use than do smaller package sizes. Unit cost is a key factor mediating this relationship. After noting useful implications for decisions regarding package size portfolios, sales promotions, and public policy, the author concludes by identifying other important but overlooked factors that increase usage volume and provide research opportunities.