Cell-penetrating peptides (CPPs) are short cationic peptides that are efficiently taken up by cells. They are able to co-internalize therapeutic molecules into cells for intracellular targets. With the increasing need for efficient carrier systems, CPPs have the potential to become an important tool to deliver therapeutic molecules across membranes. The mechanism of cell entry for the CPPs is not well understood, but the initial membrane interaction is a crucial factor for the internalization process. NR data will give a unique insight on how CPPs interact with the cell membrane and even discern the effect of fluorophore labeling on CPPs-membrane interactions. This knowledge can be of great value in the development of optimized CPPs carriers suitable for use in drug delivery systems and in understanding the consequences of using fluorophores in drug delivery efficacy assessment.