Cell-penetrating peptides (CPPs) are small molecules of major interest due to their ability to efficiently transport large cargos across cell membranes in a receptor- and energy- independent way and without being cytotoxic to cells. Since their discovery 20 years ago their potential interest in drug delivery and therapeutic diagnosis became undeniable. CPPs are being used to deliver inside cells a large variety of cargos such as proteins, DNA, antibodies, imaging agents and nanoparticle drug carriers including liposomes. Cellular uptake mechanisms of CPPs are still debated and may vary depending on their structure, nature and size of cargo they transport and type of cell line targeted. CPPs are generally rich in positively charged residues, thus they are prone to establish electrostatic interactions with the anionic membrane components (sugars and lipids). Thus understanding the molecular basis of CPP membrane interaction and cellular uptake is crucial to improve their in vivo efficiency target-specificity. A great number of studies demonstrated the high potential of CPPs to translocate efficiently therapeutic cargos into cells and some peptides are even in clinical phase studies. Although these molecules seem perfect for a therapeutic or diagnosis purpose, they still possess a small but non negligible drawback: a complete lack of cell type specificity. Tumor cells have recently been shown to overexpress certain glycosaminoglycans at the cell membrane surface and to possess a higher amount of anionic lipids in their outer leaflet than healthy cells. Such molecules confer the cell membrane an enhanced anionic character, property that could be used by CPPs to selectively target these cells. Moreover previous studies demonstrate the importance of electrostatic interactions between basic amino acids in the peptide, especially Arg residues, and the lipid headgroups and glycosaminoglycans in the cell membrane. Electrostatic interactions put at stake in this process might be one of the keys to resolve the puzzle of CPP cell type specificity.