The invention provides a peptide derivative of formula A-B-C wherein A is a polycationic nucleic acid-binding component; B is a spacer element peptide that is susceptible to cleavage within a cell; and C is a cell surface receptor binding component. The invention also provides a lipid derivative of general formula (I): (PEG)q-Linker-Spacer-Cationic headgroup-carbon skeleton-(hydrophobic chain)o wherein: the Linker is a group susceptible to cleavage within a cell; the Spacer is a group linking the Linker to the Cationic headgroup; q denotes the number of PEG chains and q=1, 2 or 3; o denotes the number of hydrophobic chains and o=1, 2 or 3; the carbon skeleton is a group linking the hydrophobic chains to the cationic headgroup. The peptide and lipid derivatives find use in non-viral gene delivery systems.