The author explores how consent functions as commitment, content, and constitution for international agreements. He argues that consent constructs all forms of international commitment. Consent elucidates an agreement’s contents – what the agreement ‘is’ in terms of scope and substance. Consent can also function as a constitution – delimiting not only ‘primary’ rules encapsulated by an agreement’s existence and contents, but ‘secondary rules’ determining who can make agreements, how they must do so, and ways to recognize, adjust, and end them. For all these functions, consent remains an under-examined and undifferentiated concept. Today, almost any of consent’s functions can be established by almost any formal or informal means. Alongside existing proposals (presumptions/defaults and content-based criteria), this chapter proposes that international law should pursue more – and different – formalities for consent. Having different forms of consent follow its different functions may, according to the author, improve the efficacy of consent and with it the efficacy of international agreements overall.