Cities are home to half the world's population, according to the United Nations, with that figure set to rise to almost two-thirds in the next 15 years.
One massive challenge that faces cities across the planet is how to lessen their environmental impact. According to the UN, cities occupy just 2 percent of the land in the world, but account for between 60 and 80 percent of energy consumption and 75 percent of carbon emissions.
One way cities can become greener is by making sure their transport links are efficient, green and easily accessible.
Here, CNBC takes a look at the past winners of the Sustainable Transport Award (STA) which, "recognizes profound leadership, vision, and achievement in sustainable transportation and urban livability."
By Anmar Frangoul, Special to CNBC.com