Peptide nucleic acids (PNAs) are polyamidic oligonucleotide analogs which have been described for the first time fifteen years ago and were immediately found to be excellent tools in binding DNA and RNA for diagnostics and gene regulation. Their use as therapeutic agents have been proposed since early studies and recent advancements in cellular delivery systems, and in the so called anti-gene strategy, makes them good candidates for drug development. The search for new chemical modification of PNAs is a very active field of research and new structures are continuously proposed. This review focuses on the recent advancements obtained by the modification of the PNA backbone, and their possible use in medicinal chemistry. In particular two classes of structurally biased PNAs are described in details: i) PNAs with acyclic structures and their helical preference, which is regulated by stereochemistry and ii) cyclic PNAs with preorganized structures, whose performances depend both on stereochemistry and on conformational constraints. The properties of these compounds are discussed in terms of affinity for nucleic acids, and several recent examples of their use in cellular or animal systems are presented.