Idylwood is a census-designated place (CDP) in Fairfax County, Virginia, United States. The population was 17,288 at the 2010 census. It originally developed as an exurban community along the route of the Washington and Old Dominion Railroad, and then as a residential suburb along Virginia State Route 7. The construction of the Capital Beltway in the 1960s, and I-66 and the Orange Line of the Washington Metrorail system in the 1980s, as well as the concurrent development of nearby Tysons Corner into Washington's leading suburban business district, led to the development of several apartment, townhouse, and small-lot single-family housing complexes, as well as the high-rise Idylwood Towers condominium, in the portion of Idylwood lying to the north of I-66. The area to the south of I-66 remains largely large-lot single-family.