I have just returned from European vacation. Here is how it works. Child up until age 2 can fly free if on lap. Of course if there is an empty seat, he/she would be able to use it at no charge. However, if you did not purchase a seat for your child, please do not bring your car seat on the plane, they will not let you use it. Of course if you did purchase a seat (discounted or full price, please bring it, it will be so much easier to fly that way. If you fly British Airways, they have what they call carry-cots which are little seats that look like bouncer seats that attach to the wall in front of you. You will still have to pay, but it is about 20% of the regular price. Buying a seat for an infant will cost you about 80% of the regular fare on BA. Hope this helps, M..