The concept of smart cities is a response to the challenges faced by cities to meet broadly shared societal objectives regarding socio-economic development, quality of life and resilience. The promise of the smart city concept is to create an environment of open, participative and citizen-centric innovation based on sharing of knowledge and resources for experimenting innovative solutions for urban development. The last twenty years witnessed a wide range of smart city concepts, experiments and practices, balancing between technological and social innovation perspectives and between corporate and citizen-centric stakeholder orientations. Growing emphasis is on creating and nurturing urban innovation ecosystems comprising an environment of cooperation, experimentation and learning rather than planning and implementation of smart city solutions. Realizing the promise of the smart city vision requires the creation of open, participative and sustainable forms of cooperation among stakeholders to breed and nurture such ecosystems as key infrastructures for urban development.