An alkyl group is a functional group in organic chemistry that is formed by removing one hydrogen from an alkane chain. The term "alkyl" is intentionally unspecific to include many possible substitutions. The general formula for an acyclic alkyl group is CnH2n+1, where n is the number of carbon atoms in the chain. A cycloalkyl group is derived from a cycloalkane by removal of a hydrogen atom from a ring and has the general formula −CnH2n−1. The smallest alkyl group is methyl, with the formula −CH3. In structural formulae, the symbol R is used to designate a generic (unspecified) alkyl group.

Alkyl groups are often used as substituents in organic molecules, and their presence can affect the physical and chemical properties of the molecule. For example, alkyl groups can increase the boiling point and solubility of a molecule. Alkyl groups can also participate in chemical reactions, such as nucleophilic substitution and elimination reactions.

Overall, alkyl groups are important functional groups in organic chemistry that are commonly used in the synthesis and study of organic molecules.