In biology, the adjective "homologous" is applied to two organisms or structures that share a (recent) common ancestor. Traditionally, the term has been applied to physical comparison (e.g., using the fossil record), but in the modern era we may commonly see nucleic acids or polypeptides of similar function being described as "homologous".

We also speak of homologous chromosomes, which are chromosomes of identical structure and highly similar DNA that form groups in polyploid organisms (pairs in diploid organisms).