Among international environmental agreements, the early climate regime gave the best illustration of the principle of common but differentiated responsibilities. The principle has been frequently invoked in the delicate negotiations on the future climate regime, and its role has gradually evolved. The 2010 Cancún Agreements promoted a type of self-differentiation which tended to blur the distinction between developing and developed countries. In the post-2020 negotiations, the notion of intended nationally determined contributions to be communicated by each party took this approach further. However, differentiation was still at the core of discussions. The Paris Agreement represents a fine balance between the requirements of differentiation and ambition. Differentiation has grown both in flexibility and adaptability. The agreement marks a decisive step forward in the gradual blurring of country categories, and better takes into account diverse national circumstances, capabilities and vulnerabilities, all of which are by their very nature changing over time.