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 (most commonly flakes). 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.

Lye is used to cure many types of food, such as lutefisk, green olives, canned mandarin oranges, hominy, lye rolls, century eggs, pretzels, as well as some types of zongzi (Chinese glutinous rice dumplings), and Chinese noodles.

Lower grades of lye are commonly used as drain openers and oven cleaners and should not be used for food preparation.