The centromeric DNA of most multicellular eukaryotes consists of tandem repeats (TR) that bind centromere-specific proteins and act as a substrate for the efficient repair of frequent double-stranded DNA breaks. Some retrotransposons target active centromeres during integration with such specificity that they can be used to deduce current and historic centromere positions. The roles of transposons in centromere function remain incompletely understood but appear to include maintaining centromere size and increasing the repeat content of neocentromeres that lack TR. Retrotransposons are known to give rise to TR. Centromere-targeting elements thus have the potential to replace centromeric TR essentially in situ, providing a mechanism to explain the centromere paradox, that is, the presence of unrelated centromeric TRs in closely related species.