The law on administrative procedure regulates the processes through which administrative decisions and administrative projects are elaborated. It is more and more regarded as essential in administrative laws: it is really considered as the central part of it in some systems. In many jurisdictions, rules concerning administrative procedure are codified, gathered in a single piece of general legislation: in a few, it remains non codified. The book is made of the different contributions presented on the topic to the last congress of the International Academy of International Law (Taipei, 2012): national reports on twenty countries and a general report. These contributions examine the way administrative procedure became codified, the obstacles which had to be overcome, the main orientations of the codes, their evolution in time; alternatively, they explain why administrative procedure is not codified. Providing extensive materials on an issue which is a concern in many administrative laws and many administrative systems, the book is intended for all searchers and experts in administrative law and public management, whether academics or practitioners.