Xenobiotic transporters are widely distributed in living organisms and play important roles in their self-defense mechanisms. Transporters cause a serious chemotherapeutic problem named multidrug resistance in pathogenic bacteria and cancer cells. Recent discoveries also support the notion that some xenobiotic transporters have shown to have important roles in bacterial virulence and signal transduction. We are interested in understanding roles xenobiotic transporters in multidrug resistance and physiological functions. Our knowledge should promote the development of novel inhibitors or strategies that could counteract the contribution of xenobiotic transporters to drug resistance and virulence.