This book deals with de minimis aid and demonstrates that it is both a sui generis legal concept in the context of State aid and subject to a complex regime. On the one hand, it is a sui generis concept in that (i) it seeks to strike a balance between simplifying the grant process and not distorting competition in the internal market, while being a tool that Member States are able to apply easily and (ii) it is subject to ex ante control by Member States. On the other hand, it is complex in that (i) it requires determining the sectors of economic activity it applies to; (ii) a few notions specific to the regime are not easy to understand, such as the notion of "single undertaking"; and (iii) it requires combining four de minimis regimes (one general and three special), which in turn requires reconciling those regimes with each other and with other aid, not least because of the cumulation rules. Lastly, these particularities were also reflected in the recovery regime for unlawful de minimis aid. Aimed at lawyers, legal consultants and those working in undertakings as well as students, the book provides a comprehensive overview of the current de minimis regimes and is clear and complete, while also proposing a fresh view on the area of EU State aid law. Ricardo Pedro is Researcher at the Centro de Investigação de Direito Público (CIDP), Universidade de Lisboa, Portugal.