First Paragraph: In this chapter I argue that some theoretical observations on the authority of legal systems that will be here discussed under the label ‘theory of de facto authority’ can shed light on an old legal phenomenon that has recently reemerged through transnational developments: the overlap between jurisdictions of law-applying bodies established by different legal systems. The chapter proceeds as follows. First, I briefly discuss the impact of transnational developments on law and legal theory and argue that some legal arrangements that are currently occurring at a transnational level are not as novel as they might seem. Then, I focus on the issue of overlapping jurisdictions and on key aspects of the theory of de facto authority to establish that this theory can account for overlapping jurisdictions, in general, and, more particularly, for their occurrence at a transnational level.