This article is a transcription of the Graduation Lecture (Maarten J. Ellis Lecture) given by the author in August 2011 on the occasion of the graduation of the 2010-2011 class of the Adv LLM Program in International Tax Law of the International Tax Center Leiden at the Leiden University, the Netherlands. The lecture was followed by comments by Claire Micheau, which are also included in this issue of European Taxation.