Peptide nucleic acid (PNA), an oligonucleotide mimic, has a non-charged achiral polyamide backbone to which the nucleobases are attached. This structure gives PNAs the capacity to hybridize with high affinity and specificity to complementary sequences of DNA and RNA and also confers remarkable resistance to DNAses and proteinases. PNAs can be conveniently delivered into cells in complex with DNA and cationic lipid. However, as with other high molecular mass drugs, the delivery of PNA, involving passage through the cell membrane, appears to be a general problem. New chemical modifications of the original PNA backbone may contribute to increasing the potentialities of PNAs and lead to the development of novel applications and PNA-dependent projects in many areas of biology and therapy. The unique physico-chemical characteristics of PNAs have led to the development of a wide range of research and diagnostic assays. Studies indicate that PNA is capable of inhibiting transcription as well as translation, so it can be used as a new tool for antigene and antisense therapy. Due to its superior properties, PNA could replace DNA as a probe for many investigation purposes. This review discusses the synthesis, chemical modifications of PNA, its important properties, cellular delivery and its major applications.