The paper focuses on the increasing recouse to principles by courts and is meant to provide for a reconstruction of the contexts in which legal principles are applied; the authority which they stem from; the aims they are used for; the meanings they have turned out to be given in the different contexts; and, finally, some results of their application. The analysis underlines how, in practice, principles tend to be used as a tool to address loose rules, values, but also sectoral trends and orientations. The analysis is carried on by looking at multiple levels - Italian domestic courts, the European Court of Justice, the European Court of Human Rights - and detects the phenomenon of the movement of principles across different legal orders, which is obviously relevant when talking about legal certainty.