Chapter 4 stresses the intrinsic international nature of air transport and travel and the consequent need to implement a common legal framework. The legal initiatives related to aviation security started to emerge in the 1960s as an answer to the first wave of hijackings and were substantiated in the Tokyo Convention in 1963. Subsequent terrorist related hijackings led to the Hague and Montreal Conventions. The consequent regulations were changed in the decade following the tragic events of September 11, 2001 by enhancing the security protocols within the aircrafts and at the airports. Such measures should be commensurate with the risks and should take into account cost-benefit ratio analyses. The chapter then considers the EU regulations for aviation security since the September 11 events. A further section of the chapter discusses the passenger name records that started as a commercial initiative and were converted to a security protocol since 2004.