Transposable elements of the DNA-mediated and RNA-mediated classes found in arthropods are briefly described and their distribution reviewed. The distribution patterns of DNA-mediated elements are extremely patchy and the principal cause appears to be the horizontal transfer of elements between host lineages. In the best documented case of mariner elements, these hosts can be in different orders of insects, classes of arthropods, and even other phyla of animals. RNA-mediated elements appear to undergo much longer periods of vertical evolution within host lineages, and evidence for their horizontal transfer remains scant. The evolutionary relationships of many of these transposons have recently been illuminated by phylogenetic analyses of the reverse-transcriptase enzymes of the RNA-mediated elements, and the recognition that the transposases of some of the DNA-mediated elements are distantly related to in the integrases of some of the RNA-mediated elements.