Corruption is a multi-faceted phenomenon, even more so when it is so deeply rooted as to be considered "normal". The problem related to defining it is strictly connected with the multiplicity of aspects that mark the phenomenon and with the difficulty of distinguishing it from other kinds of illegal acts or even from behaviors which are borderline but still legal. Here we address corruption in relationships between public and private sector, but make some reference to private sector corruption; the latter is analysed further in Part II where international treaties against corruption are addressed specifically. From a very general legal perspective, corruption can be defined as "a deterioration in the decisional process that characterizes all cases where the decider (a private or a public agent) accepts to, or asks for, deviating from the criteria that should lead his (or her) decision for his (or her) private gain; however, the reasons that lead his action cannot justify his decision" (Huber 2002). This behavioral definition focuses on key aspects of the phenomenon: the concepts of exchange of favors and abuse of power. Since our analysis is based on data collected by Transparency International (TI), we follow its definition of corruption: an abuse of public office for private gain. While acknowledging that one of the two parties needs to be a public economic agent, there is no distinction between administrative and political corruption.