This article analyses the various ways in which investment law raises questions of change. It distinguishes between changes in international investment norms, and changes in a host state’s regulatory system which is subject to the control of such norms, and explains how these different manifestations of change relate to the distinct yet interrelated issues of interpretation and application. The article explains why, given features of the contemporary investment regime, on questions of interpretation, concerning the content of international investment norms, arbitrators operate within wider processes of law-development over which states, as treaty masters, also exercise significant influence. In contrast, arbitrators dominate the process of applying international investment norms to particular investor-state disputes to determine whether changes in a host state’s regulatory system breach applicable investment norms. This claim is demonstrated in relation to the two most prominent investment treaty standards: fair and equitable treatment, and the protection against indirect expropriation.