The charges that air carriers pay for the use of airports and airways to provide international air service have become a source of recurrent and heated controversy between governments. This controversy largely stems from the rapid rise in user charges over the last decade, but it has been fed by the lack of uniformity practiced by some countries in levying these charges. Thus, the relatively specialized issue of user charges involves some important general concerns of international law, such as international economic cooperation (in the matter of arriving at general agreement on what constitutes reasonable charges), discrimination, and most-favorednation treatment.