Today’s cities are grappling with problems ranging from gridlocked traffic and overflowing landfills, to unaffordable housing and growing inequality. Without action, those problems will only get worse. The United Nations predicts that by 2050, the world’s population will reach 9.7 billion. Nearly 70% of people are projected to live in urban areas, placing a greater strain on cities and the environment. As urbanization is expected to be fastest in lower-income countries, the UN warns many cities will face challenges to meet the needs of their growing number of residents, including providing adequate housing, transportation, and energy systems. At the same time, cities are a key contributor to climate change, responsible for an estimated three quarters of global CO2 emissions. To address this, architects and designers have come up with innovative solutions – from vertical farms and biome greenhouses, to self-driving cars and underground recycling systems – to make future cities smarter and more sustainable. While some of the cities of tomorrow are still designs, others have already been built – but not always without complications. Scroll through the gallery above to discover how the cities of tomorrow are shaping up.