This chapters introduces key themes to be explored in the book. It begins by setting out the relationship between international investment law and general international law, noting that the former is a special regime that remains embedded in the general framework of international law. This embeddedness has been the focus of much attention, notably by commentators seeking to show that, notwithstanding investment law’s specialty, general international law remains relevant, to fill gaps and to provide interpretative direction. By contrast, there is relatively little debate as to whether investment law could have ‘radiating effects’, viz. influence general international law - the guiding question animating the present volume. The chapter emphasises the potential for such radiating effects: investment lawyers have, for decades, worked with general rules of State responsibility, the law of treaties, and general principles governing the resolution of international disputes; in the process, they have applied, interpreted, and often moulded the general rules. So far, general international lawyers seem to pay limited (but gradually increasing) attention to the results of the rich investment law practice. The chapter highlights the potential for investment law to radiate, and concludes with a plea for a fuller engagement, encouraging international lawyers working in other fields to tap into the rich body of investment practice.