Who wouldn't want loyal customers? Surely they should cost less to serve, they'd be willing to pay more than other customers, and they'd actively market your company by word of mouth, right? Maybe not. Careful study of the relationship between customer loyalty and profits plumbed from 16,000 customers in four companies' databases tells a different story. The authors found no evidence to support any of these claims. What they did find was that the link between customers and profitability was more complicated because customers fall into four groups, not two. Simply put: Not all loyal customers are profitable, and not all profitable customers are loyal. Traditional tools for segmenting customers do a poor job of identifying that latter group, causing companies to chase expensively after initially profitable customers who hold little promise of future profits. The authors suggest an alternative approach, based on well-established "event-history modeling" techniques, that more accurately predicts future buying probabilities. Armed with such a tool, marketers can correctly identify which customers belong in which category and market accordingly. The challenge in managing customers who are profitable but disloyal--the "butterflies"--is to milk them for as much as you can while they're buying from you. A softly-softly approach is more appropriate for the profitable customers who are likely to stay loyal--your "true friends." As for highly loyal but not very profitable customers--the "barnacles"--you need to find out if they have the potential to spend more than they currently do. And, of course, for the "strangers"--those who generate no loyalty and no profits--the answer is simple: Identify early and don't invest anything.