The purpose of this chapter is to analyse empirically how hub airports and hub-and-spoke systems have evolved over the last 15 years. First, we take a look at the cumulative distribution of the supply of flights at airports. We begin with the global distribution, and then move on to analysing regional distributions. There is a very high concentration of flights at a fairly small number of hub airports. Over the years, concentration has remained very high. These findings indicate that the hub-and-spoke system is still very effective. The majority of flights are still operated by full-service network carriers, but low-cost carriers are also starting to play an important role. Finally, we take a look at how the supply of flights in airline alliances has evolved over the last 15 years, especially in the light of the development of low-cost carriers.