With the overarching need for deep urban transformations worldwide, national platforms for cities have been emerging over the past few years in several European countries as a useful framework to support and unify the work that cities have been doing individually. In particular, Sweden and Spain have been two of the first countries where a National Cities Mission Platform has emerged, namely Viable Cities and citiES 2030. This chapter explores the emergence of these convening and intermediary vehicles, the key enablers that allowed its formation, and the rationale that consolidates it. It also delves into the distinctive value proposition of these platforms and their role in reinforcing multi-level and multi-stakeholder collaborations, facing silos in a national context, promoting cross-city stable interactions, aligning national and European initiatives toward the Cities Mission, and co-creating and consolidating the "next practices" of climate urban transitions.