ComplexCité is a federative initiative that aims to bring together the expertise available at Université Paris Cité (UPCité) and its partner institutions around the modeling of complex phenomena, spanning theoretical aspects, experimental explorations, and multidisciplinary applications. ComplexCité has been designed as a large-scale, cross-cutting structure intended to provide a framework to promote and support research and training related to the modeling of complex phenomena at UPCité. In particular, it seeks to initiate, stimulate, and develop collaborations in areas where interdisciplinary expertise, or a combination of theoretical (mathematical or computational) and experimental skills, is required.