Short retroposons (SINEs) are repetitive elements amplified in the genome via an RNA intermediate, using the enzymatic machinery of autonomous retroposons (LINEs). SINEs are widely distributed in eukaryotes; for instance, all tested mammalian genomes contain 10(4)-10(6) SINE copies. Although several SINE families such as primate Alu or rodent B1 have long been recognized, the more recent discovery of many SINEs in various eukaryotes, as well as progress in understanding the mechanisms of LINE replication and genome functioning as a whole, shed light on the biology and evolution of SINEs and their significance for the cell.