Companies are approaching this new world with caution, and with good reason: The Web can find plenty of customers for made-to-order products, but retooling a factory to spit out thousands of faultless variations on a theme is no easy job. Nike Inc. (NKE ), for example, spent six months working with suppliers in Asia to rejigger its manufacturing for custom-made sneakers. Other companies, aware of the complications, start off with reduced offerings. Staples Inc. lets customers pick fabrics or colors--but only for chairs.

Today's custom e-marketers have lots to learn from services that have flopped. One clear lesson: It's nearly impossible to customize tastes. Why? Words rarely suffice. When Generals Mills Inc. (GIS ) launched Mycereal.com two years ago, it urged breakfasters to order their dream cereals. But customers found that the taste fell far short of their mouth-watering descriptions--and cost more than $7 a box to boot. It was the same story at P&G's coffee site, Personalblends.com, which tempted coffee lovers with blends customized to their own tastes. Shoppers were stumped by the questions designed to help define their taste--and were unwilling to pay the $10 a bag that P&G needed to charge.

Still, recipes for success are emerging. Industries such as clothing that can charge a premium for quality are a natural. Others, like golf clubs and hockey sticks, that deal with simple shapes also have an edge. For businesses rooted in databases, from credit-card companies to mortgage lenders, the race to the mass market of one is well under way.