The Washington Kastles is one of eight franchises that compete in World TeamTennis. Founded in 2008 and based in Washington, D.C., the Kastles have won the WTT championship six times, tied for a league record. The team is named for Kastle Systems, a security company founded by owner Mark Ein.

Mark D. Ein is an American venture capitalist, sports team owner, and former doubles tennis player. He was born in Chevy Chase, Maryland, and earned a degree in economics from the Wharton School and a MBA from Harvard Business School. Ein worked for The Carlyle Group, Brentwood Associates, and Goldman Sachs before founding Venturehouse Group in 1999. He is a lifelong enthusiast of tennis who played in a few ATP tournaments. He is also chairman of the DC Open tennis tournament and the founder of the Washington Kastles of World TeamTennis (WTT).