By now, most people have heard of the dirt-cheap fares from low-cost carriers like Ryanair and easyJet in Europe, or Skybus and Spirit in the US. But there are some details that you really need to know.When you get a really cheap fare on a low-cost carrier, keep in mind that there are a lot of extra fees to be aware of.

Ryanair and Spirit charge between five and ten dollars per bag, and have low weight limits to boot.

Other low-cost carriers allow a checked bag or two, but have strict weight limits—and very high fees if you exceed them.

With easyJet, for example, it’s about $5 for every pound you’re overweight!

And forget about free food—on the new Skybus airline, flight attendants actually make commission for every meal they can sell to passengers.

And a growing number of airlines charge for assigned seating, and some assess reservation fees, unless you book online.

So when you’re flying low-cost, remember that you’ve got to read the fine print in order to really know what you’re getting.