Building sustainable cities Indiaâ€™s success in the 21st century is going to be defined by manner in which we address the revival of existing cities and the way we plan our new cities.According to the â€˜United Nations World Urbanization Prospectsâ€™, the next few decades alone will see 600 million Indians moving into urban areas. India's cities will have to deal with a massive influx of tens of millions of people at a speed unparalleled in history. This is clearly a challenge; but it can also be a huge opportunity to leapfrog into a society that is environmentally and socially sustainable; especially by learning from the successes and failures of the more urbanized / developed parts of the world. Here we showcase some communities that have crossed over to the sustainable side of things. A policeman walks past solar panels covering the roof of the Paul VI hall near the cupola of Saint Peter's Basilica at the Vatican The Vatican The world's smallest country and the home of the Pope is hoping to become the first solar powered nation in the world! The Vatican intends to spend 660 million dollars to create what will effectively be Europe's largest solar power plant. This massive 100 megawatt photovoltaic installation will provide enough energy to power all of its 40,000 households. People walk in St. Peter's Square at the Vatican These aren't the only moves that the Vatican is taking to reduce its greenhouse emissions. It is contemplating using an electric popemobile, the Vatican cafeteria will soon be decked with a solar heating system to provide heating and cooling, and even the Pope's summer residence is being fitted out to get power from the methane generated by the horse stables.