Earth Day, April 22, is an annual celebration of the environment and a time to assess the work still needed to protect the natural gifts of our planet.

Earth Day has no central organizing force behind it. Thousands of local events around the world mark the event.

The photographs in this gallery show the growth of Earth Day from its beginnings as an "Environmental Teach-In" in the United States in 1970, to the annual global commemoration of the natural world.