Quasi-judicial bodies have flourished in various areas of international law in the last few decades. These bodies have a mandate to monitor compliance with a body of norms, settle disputes involving those norms, or make factual determinations on the basis of investigations, yet they are not empowered to issue final, binding decisions on questions of international law. This chapter argues that quasi-judicial bodies can be viewed as international lawmakers. They act as procedural rule-makers and are developing a shared collection of principles structuring their procedures. The chapter further suggests that quasi-judicial bodies also act as lawmakers on substantive issues by influencing the interpretation, clarification and refinement of State duties and responsibilities. Quasi-judicial bodies have proved influential in the interpretation of State responsibilities by both national and international courts. Like judicial bodies, they foster compliance with international law and resolve potential conflicts.