The genome of a living being is composed of DNA sequences with diverse origins. Beyond single-copy genes, whose product has a biological function that can be inferred by experimentation, certain DNA sequences, present in a large number of copies, escape the most refined approaches aimed at elucidating their precise role. The existence of what 20th century geneticists had already perceived (and wrongly described as "junk DNA"!) was confirmed by the sequencing of the first complex genomes, including that of Homo sapiens. A large part of what defines a living thing is not unique, but repeated, sometimes a very large number of times, increasing in complexity with successive duplications and multiplication. Understanding and defining the many functions of this myriad of repeated sequences, as well as their evolution through natural selection, has become one of the major challenges for 21st century genomics.