Peptide nucleic acid (PNA) is an oligonucleotide mimic originally designed upon a repeating N-(2-aminoethyl)glycine polyamide backbone to which nucleobase heterocycles are attached through a methylene carbonyl linkage to the alpha-amino group. These molecules possess remarkable hybridization properties with DNA or RNA forming complexes with high stability and with excellent sequence discrimination despite the substantial structural divergence from natural nucleic acids. Since the disclosure of PNA, a vibrant research community with interest in the chemistry and applications of polyamide-based nucleic acid analogs has developed. This has led to the synthesis and evaluation of a wide variety of modified polyamide nucleic acids. The focus of this report is a comprehensive review of nucleobase modifications in aminoethylglycine (aeg) PNA with reference, where appropriate, to the same modification in DNA or RNA.