Alkaloids are a kind of nitrogen-containing alkaline organic compounds existing in nature (mainly plants, but some of them exist in animals), which have similar properties to alkali, so they used to be called pseudo-alkali. Most of them have complex ring structures, and most of them contain nitrogen, which has obvious biological activity and is one of the important effective components in Chinese herbal medicine. It has optical rotation. Some non-alkaline nitrogen-containing organic compounds from plants have obvious biological activities, so they are still included in the category of alkaloids. And some natural nitrogen-containing organic compounds, such as some vitamins, amino acids, peptides, etc., are not "alkaloids" by habit.

There are many kinds of known alkaloids, about 10000, and some structural formulas have not been completely determined. Their structures are complex and can be divided into 59 types. With the discovery of new alkaloids, the classification will be updated accordingly. Because there are many kinds of alkaloids with different structural formulas, their properties will be different. However, alkaloids are all nitrogen-containing organic compounds and always have some similar properties. Because amino acids are the starting substances in its biosynthetic pathway, there are mainly ornithine, lysine, phenylalanine, histidine, tryptophan and so on. They mainly undergo cyclization and carbon-nitrogen bond cleavage, so they always have some similar properties.

Alkaloids have a cyclic structure, are insoluble in water, can form salts with acids, have certain optical rotation and absorption spectrum, and are mostly bitter. Colorless crystals, some of which are liquids. There are thousands of alkaloids synthesized by different amino acids or their direct derivatives. They are one of the secondary metabolites and have toxic or strong physiological effects on organisms.