Modified nucleosides, which are derivatives of the four normal nucleosides, adenosine (A), guanosine (G), cytidine (C), and uridine (U), were found in nucleic acids as early as 1948. Modified nucleosides are contained in tRNA from all three phylogenetic domains—Archaea, Bacteria, and Eucarya—which were formerly called the kingdoms of archaebacteria, eubacteria, and eukaryotes, respectively. Although modified nucleosides are found in various positions in the tRNA, two positions, 34 and 37, contain the largest variety of modified nucleosides. This chapter presents an overview of the coding properties associated with modified nucleosides present in positions 34 and 37. Outside the anticodon, the modified nucleosides are usually “simple” modifications like methylated or thiolated derivatives, whereas all except one (archaeosine in tRNAs from Archaea) of the hypermodified nucleosides are present in the anticodon region and only in positions 34 and 37. Moreover, outside the anticodon region, only one or two kinds of modified nucleosides in each position are present, whereas a large variety of modified nucleosides are present in the anticodon region, especially in positions 34 and 37. During evolution, structural refinements of the individual tRNAs were presumably fulfilled by the evolution of the synthesis of the various modified nucleosides, including the hypermodified nucleoside. Modified nucleosides in the anticodon region exert their functions primarily in the decoding process, whereas modified nucleosides outside this region may primarily be involved in other tRNA interactions, such as interactions with translation factors or as sensors for environmental stress conditions.