An alkaloid is a naturally occurring organic compound that contains at least one nitrogen atom. They are produced by a variety of organisms, including bacteria, fungi, plants, and animals. Alkaloids have diverse and important physiological activities, including antimalarial, antiasthma, anticancer, cholinomimetic, vasodilatory, and more. Some examples of alkaloids include quinine, morphine, caffeine, strychnine, and nicotine.

Alkaloids are typically extracted from crude extracts of organisms by acid-base extraction or solvent extractions followed by silica-gel column chromatography. Certain plant families are particularly rich in alkaloids, such as the poppy family, buttercups, nightshades, and amaryllis. The function of alkaloids in plants is not yet fully understood, but it has been suggested that they may serve specific biological functions, such as protecting plants from destruction by certain insect species.

The chemical structures of alkaloids are extremely variable, but generally, an alkaloid contains at least one nitrogen atom in an amine-type structure. The name "alkaloid" was originally applied to these substances because, like the inorganic alkalis, they react with acids to form salts. Alkaloids are toxic and can have potent pharmacologic effects.