This chapter addresses the complex field of alkaloids, their definition and classification, as well as their significance in nature and industry. Despite nearly 50 years of discussion, a uniform definition for alkaloids remains difficult due to their diversity of structural features and biological sources. Previous attempts at precise classification based on chemical and structural features ultimately led to confusion. Today, alkaloids are more loosely classified as nitrogen-containing natural substances with strong physiological effects. The biosynthesis of alkaloids is distinguished based on their amino acid precursors. Approximately 10–15% of all plants contain alkaloids, from which about 9,000 to 5,000 structures have been isolated. In addition to plants, alkaloids are also present in microorganisms, such as the long-known ergot alkaloid ergotamine. The chemical structure of alkaloids is diverse but always contains at least one nitrogen atom. They can exhibit different basicities, which affects their solubility and pharmacological effect. Technically, only a few alkaloids are still of importance in medicine, although their structures have been systematically investigated to discover new drugs. Alkaloids are often biosynthesized via phenylalanine or tyrosine and can have various functions in plants, including the defense against microorganisms and herbivores. This chapter extensively discusses the biosynthesis of opium alkaloids (morphine, codeine), tropane alkaloids (scopolamine, cocaine), lysergic acids, and purines (caffeine, theophylline). For all representatives, their application, technical significance, and benefit in medicine are explained.