Multicellular organisms have evolved under relentless attacks from pathogens, and as a consequence have spiked their genomes with numerous genes that serve to thwart these threats, notably through the building of the innate and adaptive arms of the immune system. The innate immune system is by far the most ancient, being found as widely as in plants and Drosophila, while adaptive immunity arose with the emergence of cartilaginous fishes. Innate immunity enters rapidly into the game during the course of an infection and generally involves the recognition by specific cellular receptors of common pathogen-associated patterns to elicit broad defensive responses, mediated in humans by interferons, macrophages, and natural killer cells, amongst others. When innate immunity fails to eradicate the infection quickly, adaptive immune responses enter into play, to generate exquisitely specific defenses to virtually any pathogen, thanks to a quasi-infinite repertoire of nonself receptors and effectors. A specific form of innate immunity, coined "intrinsic immunity," completes this protection by providing a constant, always-on, line of defense, generally through intracellular obstacles to the replication of pathogens. This component of the immune system has gained much attention as it was discovered that it is a cornerstone of the resistance of mammals against retroviruses. One of these newly discovered intracellular molecular weapons, the APOBEC family of proteins, is active against several classes of retroelements. We present here the current state of knowledge on this rapidly evolving field and discuss implications for gene therapy.