49. This Progress by Tino Sehgal, 2010
This Progress is a performance-based piece that took place in the Guggenheim's rotunda for six weeks in 2010. The performance-experience was succinctly described in the New York Times: visitors were ushered up the infamous spiraled ramp by a series of guides, "first a child, then a teenager, then an adult, and finally an older person = who asked them questions related to the idea of progress." The show itself sold over 100,000 tickets. We'll have to wait and see if it prompts the progress its title presupposes.