Cities have become a positive and potent force for addressing sustainable economic growth, development and prosperity. They drive innovation, consumption and investment in both developed and developing countries. Cities are productive systems through which most of the 21st century challenges such as poverty, inequality, unemployment, environmental degradation, and climate change can be addressed. Owing to their unique character of population and investment agglomeration, cities link the economic, energy, environment, science, technology and social and economic elements of development, all of which constitute the key interrelations needed to formulate integrated policies required to achieve sustainable development.