We used fluorescence correlation spectroscopy (FCS) to accurately and precisely determine the relative efficiencies with which three families of "cell-penetrating peptides" traffic to the cytosol of mammalian cells. We find that certain molecules containing a "penta-arg" motif reach the cytosol, intact, with efficiencies greater than 50%. This value is at least 10-fold higher than that observed for the widely studied cationic sequence derived from HIV Tat or polyarginine Arg8, and equals that of hydrocarbon-stapled peptides that are active in cells and animals. Moreover, we show that the efficiency with which stapled peptides reach the cytosol, as determined by FCS, correlates directly with their efficacy in cell-based assays. We expect that these findings and the associated technology will aid the design of peptides, proteins, and peptide mimetics that predictably and efficiently reach the interior of mammalian cells.