Analytical legal theories are increasingly sophisticated, yet their development is uneven. While admirably adapted to explaining the unitary law-state, they are poorly suited to characterizing new law-like phenomena. From international law to the new legal order of the European Union, to shared governance and overlapping jurisdiction in transboundary areas, what at least appear to be instances of legality are at best weakly explained by approaches that presume the centrality of legal system as the mark and measure of social situations fully worthy of the title of legality. What next, as phenomena threaten to outstrip theory? Legality's Borders: An Essay in General Jurisprudence explains the rudiments of an inter-institutional theory of law, a theory which finds legality in the interaction between legal institutions, whose legality is characterized in terms of the kinds of norms they use rather than their content or system-membership. Prominent forms of legality such as the law-state and international law are then explained as particular forms of complex agglomeration of legal institutions, varying in form and complexity rather than sheer legality. This approach enables a fundamental shift in approach to the problems of identity and continuity of characteristically legal situations in social life: once legality is decoupled from legal system, the patterns of intense mutual reference amongst the legal institutions of the law-state can be seen as one justifiably prominent form of legality amongst others including overlapping forms of legality such as the European Union.