The idea of using peptides as carriers goes back some twenty years when it was discovered that the HIV-1 transactivating protein Tat is taken up by mammalian cells [30, 31] . A few years later, the Antennapedia homeodomain of Drosophila melanogaster was shown to act similarly [32] . Later on, it could be shown that peptides derived from Tat and Antennapedia as well as other proteins are capable of transporting macromolecular cargo molecules into cells [33] [34] [35] . Based on such promising results, a rapidly expanding field focusing on the so-called cell-penetrating peptides (CPPs), also referred to as protein transduction domains (PTD), began to develop. Since the first reports about Tat, a large number of naturally occurring as well as engineered CPPs have been discovered [36] [37] [38] [39] [40] [41] [42] . Table 1 gives an overview of selected "classical" CPPs. Generally, CPPs are short polycationic sequences of less than 30 amino acids that are able to translocate different cargoes (e.g. nucleic acids, peptides and even entire proteins) into cells. The only common characteristic of these peptides appears to be that they are amphipathic and net positively charged at physiological pH. Frequently the cargo is covalently attached to the CPP which can be achieved by expression as a fusion construct or by chemical coupling (for a review see: [43] ). In particular cases, cargo and carrier bind each other non-covalently through mainly ionic interactions [40, 44, 45] .