This thesis deals with the issue of mutual administrative assistance among customs authorities and its contribution to the promotion of customs cooperation. In particular, mutual assistance shall in principle be understood as the procedure to be followed in the case of (a) the exchange of information among customs authorities concerning the identity and property status of the persons involved and the origin of the controlled goods and means of transport, (b) the collection of customs duties, (c) the notification of administrative acts, (d) the conduct of joint controls against illicit trade and in favor of protection of intellectual property rights and (e) control of the origin of movement certificates. Through the reference to EU and international legislative sources and the practical problems which arise from the implementation of mutual customs assistance, it is concluded that the aforementioned procedure is trapped in rigid bureaucratic frameworks and therefore does not contribute substantially to the enhancement of customs cooperation. Therefore, solutions are proposed to improve mutual administrative assistance among customs authorities in national, EU and international level.