Being as it is volatile, multifaceted, amorphous, omnipresent, flexible, and dynamic, “practice” defies precise definition. Plural in its characteristics, practice is also plural in the roles it plays. Hence, the question arises as to how “practice” operates as a tool of treaty interpretation and application. To answer this question, it is necessary to explore the genesis and features of the “practice family” as well as the way in which species of practice operate under a treaty regime. That is crucial to ascertain a better understanding of today's role of subsequent practice in the legal regime applicable to treaties. It also allows for reflection on how the legal category of “subsequent practice” may itself evolve in the future to meet the needs of any given treaty. At present, subsequent practice is centred on the conduct of states. However, the presence of other legally relevant practice under current treaties involving various actors may require a more in-depth consideration as to how all of these different categories coexist and interact with one another.