One of contemporary art's ongoing projects has been seeking out new and innovative ways of filling the gallery space. Artists have moved from the canvas to found-object assemblages and sound installations, to video projections and architectural endeavors. As mixed-media and multimedia art slowly becomes a catch-all genre, the debate continues over what exactly defines installation art. It's clearly not just expansive sculpture — today's installation art, often directed at interventions and transformations of space, might encompass public art, institutional critiques, and curatorial practices.

Phoebe Washburn manufactures architectural meditations and self-contained sculptural systems. For While Enhancing a Diminishing Deep Down Thirst, the Juice Broke Loose (the Birth of a Soda Shop), her recent installation at the Whitney Biennial 2008, she constructed a floral ecosystem powered by Gatorade and built from two-by-fours, aquariums, tubing, and water pumps. Sarah Oppenheimer's architectural installations often consider the functions of the spaces they inhabit. Her current Horizontal Roll installation at the Saint Louis Art Museum literally creates new perspectives on the museum's collection by puncturing and reconfiguring its walls.

Urs Fischer skewered curatorial strategies for You, a recent solo show at Gavin Brown's enterprise. In a destructive institutional critique, the artist excavated the floor of the gallery, reducing it to an open pit of dirt. Artist collective Gelitin pulled a similar stunt with The Dig Cunt, in which the group dug a giant hole in a Coney Island beach every day, only to fill it back in once evening came. Fischer also teamed with Brown again to produce Who's Afraid of Jasper Johns? at Tony Shafrazi Gallery. The pair photographed the gallery's previous exhibition, Four Friends — security guards included — and plastered the images over the gallery space as wallpaper. Atop the simulacrum, Brown has hung works from Shafrazi's collection by Francis Picabia, Francis Bacon, and others. Nearby, Rob Pruitt's Viagra Falls — a Viagra-laced cascade — runs down the staircase, and Rudolf Stingel has padded the floor with white carpeting.

Another intersection in the ever-expansive category of installation art occurs where site-specific sculpture meets public art. Florian Slotawa constructs teetering works out of furniture scavenged from the institutions at which he exhibits. Meanwhile, David Byrne's Playing the Building revitalizes Lower Manhattan's nearly defunct Maritime Building, drawing large crowds eager to play an organ rigged to rattle the building's infrastructure. Mark Wallinger's interventions into public space — like the five-kilometer white thread strung through Münster at skulptur projekte münster 07 — subtly demarcate and transform their surroundings. Michael Elmgreen and Ingar Dragset's broken corporate ladder, Social Mobility, is similarly disruptive, but their work more often consists of discrete public sculpture. The pair's Prada Marfa replicates the name-brand boutique in middle-of-nowhere Texas, and a recently unveiled memorial pays tribute to the homosexual victims of the Holocaust.

As this sampling suggests, installation art is a grab bag of strategies and media, one that is neither easily defined nor exhausted. An ever-expanding category, installation art makes clear the flexibility and variety of contemporary art.

-Anna Altman