The focus of this article is the class of C
3
compounds with a central methylene group and terminal carboxylic acid, carboxylate, and nitrile functions. The high reactivity of the central methylene group, the reactivity of the acid, ester, and cyano functions, and the opportunity to build heterocycles make this class of compounds unique in the chemical synthesis. In particular the malonates, cyanoacetates, malonic acid and cyanoacetic acid, and malononitrile have all gained in importance in the life science, dyestuff, and polymer industry. This article provides general and useful technical and commercial information which can be used as a starting point for more in‐depth studies. Some other derivatives of less industrial importance are also covered.