Everything about “soft law” is controversial. International lawyers disagree about whether it is merely a “smokescreen” best used by politicians or whether it is an inevitable recourse all adjudicators need given international law’s “infinite variety.” Some have seen soft law as a fundamental threat to the rule of law, while others argue that it advances the rule of law by recognizing the complexity of its modern day sources. At the same time, there is little agreement on what exactly “soft law” is, about whether it is a useful or accurate label, and, as the quotations from Klabbers and Reisman above suggest, about whether participants in investor-state dispute settlement (ISDS) – litigants or arbitrators – should resort to it. Despite these fundamental disagreements, there is considerable evidence that, like it or not, publicly available ISDS rulings refer to what most commentators would consider to be soft law, and that in some cases, these references matter to the outcome. This essay considers what we mean by soft law, surveys its use (and possible misuse) in ISDS, and addresses how the resort to soft law relates to perceptions that the investment treaty regime or ISDS suffers from a legitimacy deficit or crisis.