Back Bay is an officially recognized neighborhood of Boston, Massachusetts, built on reclaimed land in the Charles River basin. Construction began in 1859, as the demand for luxury housing exceeded the availability in the city at the time, and the area was fully built by around 1900. It is most famous for its rows of Victorian brownstone homes—considered one of the best preserved examples of 19th-century urban design in the United States—as well as numerous architecturally significant individual buildings, and cultural institutions such as the Boston Public Library, and Boston Architectural College. Initially conceived as a residential-only area, commercial buildings were permitted from around 1890, and Back Bay now features many office buildings, including the John Hancock Tower, Boston's tallest skyscraper. It is also considered a fashionable shopping destination and home to several major hotels.

Brownstone is a brown Triassic–Jurassic sandstone that was historically a popular building material. The term is also used in the United States and Canada to refer to a townhouse clad in this or any other aesthetically similar material.