Based on the recent achievements in our lab, we have published a review entitled with “Action of antimicrobial peptides and cell-penetrating peptides on membrane potential revealed by the single GUV method” (Biophys. Rev., 12, 339-348, 2020) [Abstract]. We have developed a method to examine the effect of membrane potential on the activities of the antimicrobial peptides (AMPs) and cell-penetrating peptides (CPPs) such as membrane damage (e.g., pore formation in the membrane) and translocation across a lipid bilayer to enter the vesicle lumen using the single giant unilamellar vesicle (GUV) method. In this review, we summarized these data obtained so far. This review is based on the invited talk on a symposium at the 58th Annual Meeting of the Biophysical Society of Japan.