Peptide nucleic acid (PNA) is a nucleic acid analog in which the sugar phosphate backbone of natural nucleic acid has been replaced by a synthetic peptide backbone usually formed from N-(2-amino-ethyl)-glycine units. It is chemically stable and resistant to hydrolytic enzyme cleavage and thus not expected to be degraded inside a living cell. PNA is capable of sequence-specific recognition of DNA and RNA by hydrogen bonding and the hybrid complexes exhibit extraordinary thermal stability and unique ionic strength effects. Since its discovery, PNA has attracted major attention because of its interesting chemical, physical, and biological properties and its potential to act as an active component for diagnostic as well as pharmaceutical applications. However, the delivery of PNA, involving passage through the cell membrane, appears to be a general problem.