This special edition of urbe is the product of an ongoing initiative of the University of Geneva, UCLA and City Environment Network (ICE°NET). Within this framework two colloquia gathered colleagues from academia government and the private sector to reβlect on the increasingly complex panorama of urban transformations across the planet. In 2007, a βirst group of academics and policymakers gathered in Geneva to address new sets of challenges that mid-size cities are increasingly being subject to. We then tracked signiβicant challenges that emerging dynamics of change in respect to the environment (i.e. climate change) and energy (patterns of production and consumption) are imposing on urban areas. In addition by raising the singularity of midsize cities in terms of adaptive policy formulation and monitoring, we identiβied new barriers and enablers to mitigate the impacts of those challenges. The discussions aimed to set preliminary readings of new urban transformative trajectories.