This book attempts for the first time to engage in a comparative assessment of the different models employed in order to tackle the administrative silence in administrative law. The comparative work will be the first on this theme to tackle both the legal aspects and some empirical evidences of how the legal institution works in practice. Administrative silence occurs when administrative authority does not reply to an application in the legally prescribed time or does not take action when such action is legally prescribed. The legal fiction may be negative or positive. Negative fiction means that the law considers the silence as tacit rejection of the application and the interested parties have the possibility of a legal challenge in administrative or judicial venues. In the second instance, positive fiction, the presumption is that the silence means approval (the silent consent procedure or tacit agreement). The application is considered approved and the applicant can perform an activity. No legal system is exclusively working with just one of the legal assumptions. Both negative and positive assumptions are used, with different legal consequences.