An early "sand garden" in Boston.

Children played at a new playground on Boston Common in 1948.

A woman waved to her son as he played in the Boston Common playground in 1948.

"Playland" was an enclosed outdoor playground sponsored by the Boston Park Department, first dedicated in 1948.

Children played at the Columbia Point playground in 1968.

Mark Deegan and James Thomas at a playground in 1971.

There was a line for the slide at a playground on Mass. Ave. in 1977.

Paul McFarland, 7, and his shadow, at the new playground at Billings Field in West Roxbury in 1987.

Young children rode the swings at the Nelson Street Memorial Playground in Plymouth in 1993.

Playgrounds grew to embrace flights of artistic fancy such as sculptor Isamu Noguchi's "playscape" in Atlanta.

Some child development experts now advocate for playgrounds incorporating “loose parts,” like those at the Alexander W. Kemp Playground at Cambridge Common.

Cutting-edge playgrounds such as Denmark's Pulse Park integrate natural features with more typical "play" elements.

A play structure in Pueblo, Colo.

A playground in Hudson, Colo.

A slide in Blanca, Colo.

A slide in Cokedale, Colo.

A structure in Avondale, Colo.

A structure in Boulder, Colo.

A playground in Burlington, Colo.