Nearly 40% of mammalian genome originates from transposons, whose abundance greatly exceeds that of protein-coding genes. While historically viewed as degenerated “parasitic” DNAs, transposons yield numerous functional elements for the host genome. These sequences resulted from ancient invasion confer new mechanisms of gene regulation, generate Neogene functions, and provide raw material for genome innovation. Mammalian preimplantation embryos constitute one of the best systems to study transposon-host interactions, as 10-20% of their transcriptome results from transposon induction. Using CRISPR genome engineering, genomics, advanced imaging techniques, we aim to functionally characterize specific transposons to understand how aspects of transposon biology are repurposed for preimplantation development. Friends or foes, these ancient genome invaders are bound to generate vital insights into new developmental biology.