The immense variety of karyotypes found in extant species is unmistakable evidence that the process of evolution is associated with karyotypic change. The question whether the chromosome changes are a cause or a consequence of speciation has been debated intensely for many years and, as is often the case with biological problems, there has been no unequivocal answer. Evolution operates along different lines in different groups of organisms. In animals, reproductive biology and population structure are important factors influencing the rate of karyotypic change. Still, the most extreme chromosomal rearrangements are not necessarily found in the most specialized species. A great number of chromosome banding techniques has made it possible to study chromosomes of vertebrates in great detail. Some applications of these techniques to problems of chromosomal polymorphism in relation to mammalian speciation are presented.