This volume on the law of the European Union focuses on contemporary challenges to EU legality. Such challenges include actions or activities that cast doubt on, or sit uncomfortably with, the premises, principles, and norms that underpin the EU's legal order as proclaimed by the Treaties and the authoritative judgments of the European Court. These premises, principles, and norms range from the precisely formulated to the noticeably vague. The book develops a broader theoretical perspective as well as delving into a range of substantive areas including the Common Foreign and Security Policy, the EU's relationship with international law, migration, the sovereign debt crisis, and Brexit.