PNA (peptide nucleic acid) is a DNA mimic with a pseudopeptide backbone composed of N-(2-aminoethyl)glycine units with the nucleobases attached to the glycine nitrogen via carbonyl methylene linkers. PNA was first described in 1991 and has since then attracted broad attention within the fields of bioorganic chemistry, medicinal chemistry, physical chemistry and molecular biology due to its chemical and physical properties, in particular with regard to efficient and sequence specific binding to both single stranded RNA and DNA as well as to double stranded DNA. The present review discusses the structural features that provide the DNA mimicking properties of PNA and gives an overview of structural backbone modifications of PNA.