In the last 15 years, an ever expanding pool of cell-penetrating peptides (CPPs) has been discovered and recently focus has shifted towards improving already existing CPPs by different modifications. Since the number of published peptide sequences with cell-penetrating ability is now reaching several hundreds, the consensus methods to compare the efficacy of these is clearly needed. Many research groups are evaluating the applicability of CPPs as drug delivery vectors, all having their preferred methods of assessing uptake and intracellular distribution. Even when applying the same method, the use of different cell lines, peptide concentrations, exposure conditions, etc. are complicating comparison of data between different groups. This book is a welcome contribution to the CPP research field, hopefully paving the way for standardized protocols to be used in the future. Some of the most common methods used to this date are presented and compared in this chapter.
