Germ-line transformation of insects is now possible with four independent transposable element vector systems. Among these, the TTAA-insertion site specific transposon, piggyBac, discovered in Trichoplusia ni, is one of the most widely used. Transformations have been achieved in a wide variety of dipterans, lepidopterans, and a coleopteran, and for many species, piggyBac transposition was first tested by plasmid-based mobility assays in cell lines and embryos. All plasmid and genomic insertions are consistent with the duplication of a TTAA insertion site, and most germ-line integrations appear to be stable, though this is largely based on stable marker phenotypes. Of the vector systems presently in use for non-drosophilids, piggyBac is the only one not currently associated with a superfamily of transposable elements, though other elements exist that share its TTAA insertion site specificity. While functional piggyBac elements have only been isolated from T. ni, nearly identical elements have been discovered in a dipteran species, Bactrocera dorsalis, and closely related elements exist in another moth species, Spodoptera frugiperda. It appears that piggyBac has recently traversed insect orders by horizontal transmission, possibly mediated by a baculovirus or other viral system. This interspecies movement has important implications for the practical use of piggyBac to create transgenic insect strains for field release.