As an increasingly urban planet, our major environmental challenges, including water conservation, climate change and loss of biodiversity, are now urban problems. This makes the quest for green cities essential for human survival and the health of our planet. As more and more people live in cities, the activities and behaviors that cities encourage will determine whether we make the planet less or more habitable.

Green cities enable the people who live and work in them to carry out their daily lives in an environmentally sound manner. Many of the negative impacts of urban living occur through everyday activities, such as how we move about what we eat, and how we make our homes and workplaces comfortable. These decisions are influenced by urban design and public policies, so green cities attempt to change the landscape to make commuting, feeding their families and tossing our trash a part of an overall ecological plan.

When deciding just how green a city really is, look for the air quality, water quality, amount of green space in public areas, percentages of residents who walk, bike or use public transit, and the availability of locally grown food. The following cities all ranked high in these areas.

Billings, Montana is by far one of the greenest cities in the Midwest. The city initiated the Trash into Trees program to keep several tons of paper and aluminum cans out of its landfills, and used the money earned from recycling them to plant thousands of trees around Billings. The city is home to the Northern Plains Research Council, which is housed in one of the most eco-friendly buildings in the US.

In Seattle, Washington, all new constructions within the city limits must be certified as LEED Silver or better, ensuring that all buildings in the city will be truly green. The city hosted the US Conference of Mayors Climate Protection Agreement, urging cities nationwide to cut CO2 emissions by 7 percent over the next five years. Every decision that the mayor’s office makes, from choice of paper to location of meetings, is made with the environment in mind.

Portland, Maine, has topped several lists of Greenest Cities, with good reason. The city has incorporated thousands of acres of greenbelts, parks and trails. The city’s public transit and all work vehicles run on a mixture of diesel and biodiesel, and have strict anti-idling mandates. The city even hosts the largest organic fair in the world, the annual Common Ground Country Fair.

Housing one of the country’s greenest buildings, the headquarters for Heifer International, Little Rock, Arkansas might be a surprising addition to any list of green cities. But when you consider the city’s plan to build a green park inside very eight blocks and the more than 6,000 acres of public park space already available there, you’ll see why it makes our list.

Cities nationwide are working to improve their green ratings and make a difference for their residents. As urban areas continue to grow, so does their impact on the earth and environment. By thoughtfully controlling growth and working to build greener buildings and spaces, these green cities are ahead of the pack so far.