Throughout comparative administrative law there is a concept that is not unequivocally labelled: public power, administrative power or public function. Nor is its semantic load the same in each legal system. This is why it is so difficult to translate it into English, as reflected in the title of this monograph. Nevertheless, in different legal systems this term is of increasing importance in determining the scope of application of administrative law: it is used to identify bodies, tasks or functions that must be subject to specific rules of application and control, laid down by administrative law; in some cases, regarding relationships between privates. This monograph analyses the concept of administrative public power in several European legal systems: firstly, to know the scope of the concept in each legal system, and secondly, to see how it operates as a criterion for the application of administrative law. Although it may seem a conceptual and strictly academic subject, this book is very relevant for the interpretation and practical application of the law, due to the consequences that the concept of public administrative power has for delimiting the legal regime of many activities, both in the public and private sectors.