This review deals with 2-azapurine (imidazo[4,5-d] [1,2,3]triazine) nucleosides and closely related analogs. Different routes are described to yield the desired target compounds, including a sequence of ring-opening and ring-closure reactions performed on purine nucleosides or direct glycosylation of a 2-azapurine nucleobase with a sugar halide. Further, physical and spectroscopic properties of 2-azapurine nucleosides are discussed, including fluorescence, 13C-NMR data, single-crystal X-ray analyses, and conformation studies on selected compounds; new biological data are presented. The second part of this review is dedicated to oligonucleotides containing 2-azapurines, including building-block (phosphoramidite) preparation and their use in solid-phase oligonucleotide synthesis. Base-pairing properties of 2-azapurine nucleosides as surrogates of canonical constituents of DNA were evaluated.