This book explores the tools that the European rules on State aid place in the hands of competitors when it comes to fighting subsidies and other state measures of financial assistance to firms. In order to do so, the book scrutinises the means of redress available to competitors before national courts (private enforcement), as well as the opportunities that they have to make their voice heard in the course of the European Commission's enforcement procedures (public enforcement). The insights provided by the book lead to a better understanding of the rights of private parties under the rules and practices that govern the enforcement of State aid law.