The recent expansion of international law, the proliferation of international organizations and international courts and tribunals pose new challenges to international law scholars and require new perspectives. This chapter explores what role, if any, constitutional law and jurisprudence can play in the development of international investment law. In examining the various forms of this interaction between international investment law and constitutional theory, the chapter demonstrates that while constitutional theory certainly constitutes a useful approach to study international investment law, it does not constitute the sole or the ultimate method for approaching the same. Rather, the chapter suggests, international investment law requires 'epistemological pluralism,' that is, different methods of enquiry. Only the juxtaposition of different methods and approaches can help scholars and practitioners to decipher the complexity of international investment law.