This thesis intends to analyze and clarify the relevant market within air passenger transport. Further the thesis intends to investigate if there is any difference in air passenger transport in accordance to more general business. The definition of the relevant market is an essential step in order to establish a breach under European competition law. The relevant market includes the relevant product market and the relevant geographic market. The fundamental issue in the relevant product market is whether products are considered to be substitutable and can constitute the same market. The Commission has set a notice which includes methods and criterias and serves as guidelines to courts and companies to define the relevant market. The notice is however not binding for courts. The CJ have developed implementation of the criterias which it tends to refer to even if it is a different branch, however a case-by-case based definition is needed. The definition of the relevant market is based on three main criterias: demand substitutability, supply substitutability and potential competition. Even though the supply substitutability seems not be implemented in a further extension in more general branches, it seems to be of greater importance when defining the relevant market within air passenger transport. The air passenger transport has also established a test called base of origin and base of destination which includes an analysis of price, travel frequencies, comfort of the journey, transfer time to terminals and differences in the qualities and quantities in airports.  However according to air passenger transport, travel sequences seems to be the criteria that courts focus most on.