Tandemly repetitive DNA is a major component of all eukaryotic genomes. This fact has been known for almost 30 years and research on this class of DNA is still being done. Its biology and evolution are therefore now becoming fairly well understood. DNA-fingerprint techniques rely very much on this knowledge. However, the large amount of research on these sequences has inevitably led to a large number of different concepts and theories about their nature. This has also resulted in some confusion as to the nomenclature. The following notes are intended to resolve this confusion somewhat and to give some definitions for the major classes of tandemly repetitive DNA.