Glenstone is a private contemporary art museum in Potomac, Maryland, 15 miles (24 km) from downtown Washington, D.C. The museum's exhibitions are drawn from a collection of about 1,300 works from post-World War II artists around the world. It is the largest private contemporary art museum in the United States, and is noted for its setting in a broad natural landscape. The museum was developed and financed by billionaire American businessman Mitchell Rales, and is curated by Emily Wei Rales. Artists in Glenstone's collection are required to have already been exhibited for at least 15 years. First opened in 2006 in a building designed by Charles Gwathmey, the museum has been expanded several times between 2013 and 2018 on its 230-acre (93 ha) campus. Its largest expansion was opened to the public on October 4, 2018, with outdoor sculpture installations, landscaping, and the introduction of a new museum complex called the Pavilions designed by Thomas Phifer. In 2019, Glenstone added an environmental center with exhibits on recycling, composting, and reforestation. Glenstone has been compared to other private museums such as the Frick Collection and The Phillips Collection, and has received generally positive reviews in the press. The museum is open, free to the public, via online booking.