The proton-dependent oligopeptide transporters (PEPTs, SLC15) and the organic cation transporters (OCTs, SLC22), two mechanistically very distinct families within the Major Facilitator Superfamily (Transporter Classification Database, www.tcdb.org), share a common thread: a broad substrate selectivity, which in recent years has been subject to intense scrutiny, driven by the prospect of using these carriers for the delivery of drugs and prodrugs. Their relatively tolerant recognition and binding requirements, together with their localization to barrier epithelia and organs of drug disposition, make the PEPTs and the OCTs prime candidates for rational drug design, but these same properties can lead to competitive drug-drug interactions or toxicity in non-target organs. Pharmacological, structural and computational studies contribute to the refinement of existing pharmacophore models, towards improved targeting strategies and the prevention of undesired side effects. PEPT1 (SLC15A1) and PEPT2 (SLC15A2) are mainly expressed, respectively, in the brush-border membrane of enterocytes, and in the apical membrane of renal proximal tubular cells, the lung epithelium and the choroid plexus, where they mediate the absorption, reabsorption and accumulation of all di- and tripeptides resulting from the hydrolysis of dietary and endogenous protein. In addition, they have been shown to recognize a broad spectrum of pharmacologically active compounds, both peptidomimetic, such as the β-lactam antibiotics, and non-peptidic. By means of biochemical and electrophysiological assays in Xenopus oocytes, we showed that the widely-prescribed antibiotics ampicillin, amoxicillin, cephalexin, cefadroxil, and the antineoplastics bestatin and δ-aminolevulinic acid are transported by human PEPT1 and PEPT2, and we established the alternating-access mechanism by which both oligopeptides and xenobiotics are translocated (Sala-Rabanal, Loo et al. 2006, Sala-Rabanal, Loo et al. 2008). The drugs, however, were transported with much lower efficiency than the natural substrates, and our findings suggest that the presence of physiological concentrations of dietary peptides in the gut may reduce or delay the absorption of oral delivery drugs, which should thus be taken in an empty stomach. In humans, OCT1 (SLC22A1) and OCT2 (SLC22A2) are predominantly expressed in hepatocytes and proximal tubular cells, whereas OCT3 (SLC22A3) is found in placenta, bronchial and intestinal epithelium, and astrocytes. OCTs interact with many natural and xenobiotic monovalent cations and have been reported to transport dicationic compounds, including the short polyamine putrescine. In a recent study, we used Xenopus oocytes expressing mammalian OCT1, OCT2, or OCT3 to demonstrate that spermidine, which is longer and carries an additional positive charge, is also transported by the OCTs; and that, as for monovalent cations, hydrophobicity is a major requirement for recognition in polyvalent OCT substrates and inhibitors (Sala-Rabanal, Li et al. 2013). The identification of OCTs as relevant polyamine exchange systems may contribute to further our understanding of the physiological roles of polyamines, and aid in the design of polyamine-like OCT-targeted drugs: carcinogenesis and tumor growth have been associated with increased intracellular polyamine levels, and thus the OCTs might be targeted for the delivery of cytotoxic polyamine analogs or polyamine-conjugated imaging probes.