Following recent decisions in Sanum v Laos and World Wide Minerals v Kazakhstan, investment lawyers have begun to engage with the legal rules governing State succession to treaties. As State succession is one of the more technical and controversial areas of general international law, this engagement can present challenges; however, the issues are too important to be ignored. This article maps out the most pressing questions of State succession that investment lawyers have faced, or are likely to face in the future. It identifies the three most salient problems - viz the succession of new States to ICSID membership and to old BITs, and the impact of cession of territory on investment protection. With respect to each of these three problems, the article analyses the general regime of State succession and its application to the investment law context, highlighting uncertainties in the law and proposing ways of dealing with them.