 Cell-penetrating peptides, CPPs, are short amino acid sequences that can be used to deliver large biomolecules into cells. These peptides are positively charged at physiological pH and can pass through cell membranes by either endocytosis or direct penetration. The exact mechanism of CPP-mediated cellular uptake depends on the peptides concentration, cargo, and the cell type. This article was authored by Ivana Ruska and Andrea Simmer.