The collection of contributions to this issue of Erasmus Law Review illustrates the breadth of the academic writing on the theme of precaution. This broadening of academic interest in precaution can be explained by the fact that the application of the PP is often highly contested. The European Union’s claim that the PP is a received principle of international environmental law is problematic, at least in the sense that questions of when and how to apply it always give rise to considerable controversy. The PP can therefore be considered a ‘contested concept’