One of the salient features of the mammalian genome is the vast excess of DNA without obvious function, such as repetitive DNAs, spacers, and introns. In recent years, microsatellites, which include short triplet repeats (mostly CAGn and CGGn) and dinucleotide repeats (notably CAn) have gained widespread attention, along with minisatellites which consist of somewhat longer repeat units. Micro- and minisatellites, collectively called variable number tandem repeats (VNTRs), can be highly unstable and display an amazing degree of polymorphism. This property is exploited for gene mapping, for tumor diagnosis, and in forensic medicine. Undue expansion of gene-associated microsatellites is also responsible for some severe genetic diseases, such as fragile X syndrome. Most or all of these diseases are caused by expansion of CAG and CGG triplets. Within protein-coding regions these triplets usually code for polymers of glutamine, serine, alanine or proline. Physiologically, such amino acid repeats are often found in transcription factors and can increase or decrease their activity, depending on the repeat number. Alone or in conjunction with DNA methylation, such repeats may offer a unique opportunity for subtle, semi-stable modulation of gene activity. Also, at least in some plants and perhaps other organisms, a quasi-Lamarckian inheritance is mediated by repetitive DNA. Generally, repetitive DNA sequences, whether represented by short or by long DNA segments, may be beneficial for the evolution of a species.