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.