You count them. You hoard them. You check your statement compulsively to see just how close you are to getting that flight to Hawaii. You're not the only one. There are some 67 million travelers in the United States alone who are members of a frequent-flier program, and there are 100 million total worldwide. All told, there are about 3.5 trillion miles yet to be redeemed.

Miles are the most powerful promotional currency on the face of the planet. How did they get so popular? "The mile is simply what evolved out of discussions about devising the optimum retention and customer-motivation tool," says Robert Crandall, the retired chairman of American Airlines. Along with his management team, Crandall invented the concept, which was introduced in 1981. "We thought that we could come up with an equivalent to the S&H Green Stamp -- only better. Because we automated it, customers didn't have to do anything to collect the miles except fly on American." By the end of 1981, 1 million people had signed up for the AAdvantage program. Today, it has about 43 million members.

The tipping point for the mile, however, came when companies outside the airline industry decided to give airline miles to their loyal customers. It started with hotel chains and rental-car companies, then moved to the credit-card industry and phone companies. By 1999, 40% of the miles that people earned (not counting certain bonuses for very frequent fliers) didn't come from flying at all. The airlines sold about $2 billion worth of miles to other companies to offer as incentives last year. People actually fight over miles in divorce court.

With so many miles in circulation, you can't help but wonder if a currency devaluation is imminent. Randy Petersen, who has built a business in Colorado Springs called Frequent Flyer Services around helping people maximize their miles, figures that consumers will net another 500 billion miles by the end of this year. Last year, people turned around and cashed in those points for more than 13 million airplane tickets.

The basic problem is that people are earning miles a lot faster than airlines are buying planes and adding routes to their networks. What does the future hold for frequent-flier programs? Here are answers to seven high-flying questions.

1. Today, most domestic award tickets in the United States require a minimum of 25,000 miles. Will that minimum go up?

At some point, it has to -- although not right away. After all, the last time American raised the mileage minimum, members of AAdvantage filed suit against the airline, claiming that it had not properly notified them. Richard G. Barlow, chairman and CEO of Cincinnati-based Frequency Marketing Inc., which develops loyalty programs for companies such as American Express, GE Capital, and Microsoft, thinks that the airlines would prefer to raise the prices that they charge to the credit-card companies and hotel chains, which purchase the miles to give to their own customers. If those companies give fewer miles away as a result, so be it. It's better for the airlines to disappoint Hertz's and Hilton's customers than to raise the mileage threshold for the people who actually fly in their planes.

2. Is it going to become easier to use miles to get to really popular destinations (can you say Hawaii)?