Lysergic acid diethylamide, abbreviated LSD or LSD-25, also known as lysergide and colloquially as acid, is a semisynthetic psychedelic drug of the ergoline family, well known for its psychological effects which can include altered thinking processes, closed and open eye visuals, synaesthesia, an altered sense of time and spiritual experiences, as well as for its key role in 1960s counterculture. It is used mainly as an entheogen, recreational drug, and as an agent in psychedelic therapy. LSD is non-addictive, is not known to cause brain damage, and has extremely low toxicity relative to dose, although in rare cases adverse psychiatric reactions such as anxiety or delusions are possible.

LSD was first synthesized by Albert Hofmann in 1938 from ergotamine, a chemical derived by Arthur Stoll from ergot, a grain fungus that typically grows on rye. The short form "LSD" comes from its early code name LSD-25, which is an abbreviation for the German "Lysergsäure-diethylamid" followed by a sequential number. LSD is sensitive to oxygen, ultraviolet light, and chlorine, especially in solution, though its potency may last for years if it is stored away from light and moisture at low temperature. In pure form it is a colorless, odorless, and mildly bitter solid. LSD is typically delivered orally, usually on a substrate such as absorbent blotter paper, a sugar cube, or gelatin. In its liquid form, it can also be administered by intramuscular or intravenous injection. LSD is very potent, with 20–30 µg (micrograms) being the threshold dose. New experiments with LSD have started in 2009 for the first time in 40 years.

Lysergic acid, also known as D-lysergic acid and (+)-lysergic acid, is a precursor for a wide range of ergoline alkaloids that are produced by the ergot fungus and some plants. Amides of lysergic acid, lysergamides, are widely used as pharmaceuticals and as psychedelic drugs (LSD). Lysergic acid received its name as it was a product of the lysis of various ergot alkaloids .

Lysergic acid is generally produced by hydrolysis of natural lysergamides, but can also be synthesized in the laboratory by a complex total synthesis for example by Woodward's team in 1956 . An enantioselective total synthesis based on a palladium catalyzed domino cyclization reaction has been described in 2011 by Fujii and Ohno. Lysergic acid monohydrate crystallizes in very thin hexagonal leaflets when recrystallized from water. Lysergic acid monohydrate, when dried (140 °C at 2 mmHg or 270 Pa) forms anhydrous lysergic acid. The biosynthetic route is based on the alkylation of the amino acid Tryptophan with dimethylallyl diphosphate (isoprene derived from 3R-mevalonic acid) giving 4-dimethylallyl-L-tryptophan which is N-methylated with S-adenosyl-L-methionine. Oxidative ring closure followed by decarboxylation, reduction, cyclization, oxidation, and allylic isomerization yields D-(+)-lysergic acid.

Albert Hofmann (January 11, 1906 – April 29, 2008) was a Swiss scientist known best for being the first person to synthesize, ingest and learn of the psychedelic effects of lysergic acid diethylamide (LSD). He authored more than 100 scientific articles and a number of books, including LSD: My Problem Child. He was named #1 out of 100 in Telegraph's magazine 2007 "Top 100 Living Geniuses" list.

Hofmann was born in Baden, Switzerland, the first of four children to factory toolmaker Adolf Hofmann and his wife Elisabeth (born Elisabeth Schenk). Owing to his father's low income, Albert's godfather paid for his education. When his father became ill, Hofmann obtained a position as a commercial apprentice in concurrence with his studies. At the age of twenty, Hofmann began his chemistry degree at the University of Zürich, finishing three years later, in 1929. His main interest was the chemistry of plants and animals, and he later conducted important research on the chemical structure of the common animal substance chitin, for which he received his doctorate, with distinction, in 1930.