Mobile DNAs have had a central role in shaping our genome. More than half of our DNA is comprised of interspersed repeats resulting from replicative copy and paste events of retrotransposons. Although most are fixed, incapable of templating new copies, there are important exceptions to retrotransposon quiescence. De novo insertions cause genetic diseases and cancers, though reliably detecting these occurrences has been difficult. New technologies aimed at uncovering polymorphic insertions reveal that mobile DNAs provide a substantial and dynamic source of structural variation. Key questions going forward include how and how much new transposition events affect human health and disease.