This chapter focuses on a particular aspect of treaty practice — the conclusion of a treaty and evolution of a treaty regime in practice. It takes as a starting point the rules codified in the 1969 Vienna Convention on the Law of Treaties. It also considers ways in which the conclusion of treaties and the evolution of treaty regimes transcend the 1969 VCLT. One of this chapter’s concerns is for those dimensions of lawmaking by treaty where no element of consent is expressed in the way normally required under the VCLT. Having discussed the position reflected in the 1969 VCLT it proceeds to examine the bold, indeed at times controversial, activities of treaty bodies including conferences of the parties (COPs) and meetings of the parties (MOPs) in the regime of multilateral environmental agreements (MEAs), as well as the evolutionary interpretation and alleged activism by international courts. In this chapter both of these practices under multilateral treaties are considered as free-standing processes for the development of a treaty regime, that is, as distinct from subsequent agreement or subsequent practice of the parties as a means of interpretation of a treaty under article 31(3)(a) and (b) VCLT.