This paper deals with public service obligation, a form of state aid that applies to air services. The paper first provides general information on the European legislation applying to this form of state aid, and elaborates the legal framework and general principles. The second part is dedicated to a comparison of a similar subsidizing programme in the USA and Australia. An examination of current imposed public service obligation routes in Europe is provided in the following section. The coefficients defining the number of imposed PSO routes per various geo-economic variables have been defined.