Airline alliances dominate in the air transport industry and approximately the two thirds of global passengers fly with one of the three biggest alliances. Consequently, it is of paramount importance for an alliance to offer aligned services across its members to satisfy their passengers' needs. This book analyses the ancillary services and how their introduction changed the aviation industry. Moreover, are scrutinized the ancillary services included in the most restricted economy and business fares of the full-service carriers, who participate in one of the three biggest alliances. In addition the alliances and their members are classified into three categories and are inspected possible incompatibilities. Furthermore, a Risk Compatibility Index (RCI) is calculated, which shows the possibility of a connecting passenger of a codeshare flight to have different ancillary services in at least one leg of his entire trip. To further support the analysis, a route case was selected and examined which alliance has the highest possibility to offer different services to its connecting passengers.