The áo dài (pronounced 'ao yai' in the south; pronounced 'ao zai' in the north) is a traditional Vietnamese dress worn by women. It is the uniform for female students in Vietnamese secondary schools and universities. The áo dài is and has been more prevalent in southern Vietnam than in northern Vietnam.

After 1975, the áo dài was rarely seen because many considered it to be an excess inappropriate for hard work. However, since the 1990s, the áo dài has seen a resurgence.

Early versions of the áo dài date back to the 1700s. In 1744, Lord Vũ Vương of the Nguyễn Dynasty declared that men and women should wear trousers and a gown with buttons down the front.

The modern version of the áo dài did not appear until 1930, when Vietnamese fashion designer Cat Tuong, known to the French as Monsieur Le Mur, modified it. He lengthed the áo dài so that the top reached the floor, made it fit the curves of the body closer and moved the buttons from the front to an opening along the shoulder and side seam.

Men began to wear the áo dài less and less, except at certain ceremonies such as weddings and funerals.

In Saigon during the 1950s, Tran Kim of Thiet Lap Tailors and Dung of Dung Tailors modified the áo dài to the form seen today. He produced the gowns with raglan sleeves, creating a diagonal seam that runs from the collar to the underarm. This is the version of the áo dài preferred by most today.

The áo dài has been popular among many Vietnamese Americans since 1982, when Tram Kim opened a branch of Thiet Lap Tailors in Garden Grove, California.