Lye is a corrosive alkaline substance, commonly sodium hydroxide (NaOH, also known as 'caustic soda') or historically potassium hydroxide (KOH, from hydrated potash). Previously, lye was among the many different alkalis leached from hardwood ashes. Today lye is commercially manufactured using a membrane cell method.

Solid dry lye is commonly available as flakes, pellets, microbeads, and coarse powder. It is also available as solution, often dissolved in water. Lye is valued for its use in food preparation, soap making, biodiesel production, and household uses, such as oven cleaner and drain opener.

It is also used as an ingredient in the production of methamphetamine. Thus, keeping large quantities of lye is controlled or regulated in some areas.

Lye is used to cure many types of food, such as lutefisk, green olives, canned mandarin oranges, hominy, lye rolls, century eggs, and pretzels. It is also used as a tenderizer in the crust of baked Cantonese moon cakes, and in lye-water "zongzi" (glutenous rice dumplings wrapped in bamboo leaves), in chewy southern Chinese noodles popular in Hong Kong and southern China, and in Japanese ramen. In the United States, food-grade lye must meet the requirements outlined in the Food Chemicals Codex (FCC), as prescribed by the U.S. Food and Drug Administration (FDA). Lower grades of lye are commonly used as drain openers and oven cleaners and should not be used for food preparation.

Len Lye, born Leonard Charles Huia Lye (5 July 1901 - 15 May 1980), was a Christchurch, New Zealand-born artist known primarily for his experimental films and kinetic sculpture. His films are held in archives such as the New Zealand Film Archive, British Film Institute, Museum of Modern Art in New York City, and the Pacific Film Archive at University of California, Berkeley. Lye's sculptures are found in the collections of the Whitney Museum of American Art, the Art Institute of Chicago, the Albright-Knox Art Gallery and the Berkeley Art Museum. Although he became a naturalized citizen of the United States in 1950, much of his work went to New Zealand after his death, where it is housed at the Govett-Brewster Art Gallery in New Plymouth.

As a student, Lye became convinced that motion could be part of the language of art, leading him to early (and now lost) experiments with kinetic sculpture, as well as a desire to make film. Lye was also one of the first Pākehā artists to appreciate the art of Māori, Australian Aboriginal, Pacific Island and African cultures, and this had great influence on his work. In the early 1920s Lye travelled widely in the South Pacific. He spent extended periods in Australia and Samoa, where he was expelled by the New Zealand colonial administration for living within an indigenous community.