The second most costly part of a tour, after lodging, is airfare. Airfares were sky-high a few years ago and it is my opinion the legacy carriers like American, Delta and United were gouging the consumer, despite lower fuel costs, on their fares to Europe. It was my prediction that other "discount" airlines would come along and take advantage of a lucrative market by offering lower-fares to a variety of destinations in Europe. But be aware, not all discount airlines are the same. In this article, The Traveling Professor names his favorite discount airlines to Europe.