Marketing is typically seen as a tool for growth. A company can use it to successfully launch a product, make inroads into a new market, or gain share with existing products in its current market. But for nearly every new product launch, market entrant, or industry upstart grabbing market share, there is an incumbent that must defend its position. If the defender can’t hang on to what it has, it loses the foundation on which to build its own growth.

A version of this article appeared in the November 2005 issue of Harvard Business Review.