Here's advance warning. If you're flying somewhere this week and checking luggage, be prepared for extra delays.

Beginning this week, most economy-class passengers on domestic flights operated by US Airways, Continental, Delta, Northwest and United will have to pay $25 extra if they want to check a second piece of luggage.

Other carriers, including American Airlines, Air Canada and AirTran, will introduce similar fees later this month. JetBlue is to impose a $20-second-bag fee June 1.

Until now, these carriers permitted two free checked bags. The change is certain to cause considerable problems for passengers, not to mention extra charges. For now, Midwest, Southwest and USA 3000 will continue their policies of two free checked bags per customer.

While the specifics vary from airline to airline, the new checked bag policies generally apply only to passengers traveling in economy class, or on tickets that are discounted or free. Also, the $25 fee applies to the second bag; additional bags can cost up to $100 each.

The rules are unchanged for travelers ticketed in first or business class, as well as those with elite status on that carrier's frequent flier program, unaccompanied minors and military personnel traveling on active duty. The checked bag allowances also stay the same for passengers taking international flights, usually two free pieces, but US Airways is applying the new one-free-bag rule to its flights to the Caribbean and Mexico.

The charges are per bag each way for domestic flights, so a second bag to check will cost at least $50 extra round trip. All additional baggage charges are due on the spot and must be paid with a credit or debit card, cash or check. And since it's not possible to pay the extra bag fee in advance, be prepared to have to go through the process both ways.

Additional bag fees also apply to sports equipment. That's why US Airways recommends that passengers consider using alternative shipping services, such as Sports Express (sportsexpress.com), to transport equipment, albeit at considerable extra cost. For example, Sports Express quotes a charge of $193.29 each way to transport a large golf bag between Pittsburgh and Phoenix.

But don't think about getting around the limitation by packing one bigger one. Carriers around the globe have begun enforcing their limits on bag size and volume.

Charges for luggage exceeding those specifications can range up to $100 per piece each way.

The new rules will also affect business travelers who regularly take sample cases, merchandise displays or other bulky items on their trips.