Transposable elements are a polyphyletic group of repeated sequences that constitute a significant fraction of our genome and are able to replicate and introduce new copies through insertional mutagenesis. This mutagenic potential, as well as other deleterious effects derived from their replication and repetitive nature, has led to considering them as selfish DNA or molecular parasites. However, there is mounting evidence of their positive contribution to the adaptation and evolution of their hosts. This chapter aims to describe paradigmatic examples of these frenemies that have contributed to very important features in current living organisms, including the coordination of embryonic gene programs, the defense against retroviruses, the capture of endosymbiotic organelles, the adaptive immune system, and even the lack of a tail in humans.