sequence [14; 15]. Many modified nucleotides are implicated in the formation of the tertiary interactions that lead to the L-shaped structure of tRNAs (see, for instance, the position of modified nucleotides in _E. coli_ and _S. cerevisiae_--Figure 1c,d) and are thus key to tRNA stability [16; 17] and resistance to RNA decay pathways [18; 19; 20; 21]. Based on their position in the three-dimensional structure, modifications can be divided into two groups, namely modifications in the tRNA core and modifications in the ACL region (Figure 1). Figure 1: tRNA structure and post-transcriptional modifications: (**a**) Cloverleaf representation of tRNA secondary structure. tRNAs are composed of five main regions, namely the acceptor stem (in purple) with the CCA-end (in grey), the D-arm consisting of the D-loop and D-stem (in blue), the anticodon-arm (in green), the variable region (in yellow), and the T-arm consisting of the T-loop and T-stem (in red). Residue numbers are according to the standard tRNA nomenclature [22]. (**b**) Schematic representation of the L-shaped tertiary structure of tRNA with the same color codes as in (**a**). The acceptor-stem stacks on the T-arm, and the D-arm stacks on the anticodon-arm. The T-loop and D-loop interact and form the tRNA elbow structure. Main tertiary interactions between the T- and D-loops and between the variable region and the D-stem are represented with dashed lines, and the positions of the implicated nucleotides are indicated. (**c**) Locations of modifications of modifications in _Escherichia coli_ tRNAs. (**d**) Locations of modifications in _Saccharomyces cerevisiae_ tRNAs. Data are compiled from the MODOMICS database [14] and references [23; 24] for _E. coli_ tRNAs and [1; 2] for _S. cerevisiae_ tRNAs. Modified positions are represented as red circles on the schematic view of the L-shaped tertiary structure of tRNA. Residue numbers and modifications found in _E. coli_ and _S. cerevisiae_ are given for each modified position. The implication of ACL modifications in the decoding process has been extensively studied, providing many details on the function and dysfunction associated with these modifications (reviewed in [25; 26; 27; 28; 29]). The ACL contains complex tRNA modifications that are involved in codon recognition (e.g., at wobble position 34) and reading frame maintenance (e.g., at position 37) [30; 31; 32; 33; 34; 35; 36; 37]. Modifications at the wobble position can either restrict or expand the decoding properties of the tRNA, and modifications in the ACL are thus intricately linked to the efficiency and accuracy of translation. Thus, modifications in the ACL can modulate the expression of specific genes that are enriched in codons decoded by tRNAs modified at the wobble position. This notion of modification-tunable expression of codon-biased genes allows for an additional layer of regulation in the gene expression process, which may have general implications for cellular physiology and stress adaptation (reviewed in [38; 39; 40; 41]). Modifications in the tRNA core, i.e., outside the ACL region, are also implicated in several important cellular functions, such as translation, fitness and stress adaptation. However, the molecular mechanisms involved are generally less well characterized and, therefore, more difficult to comprehend than those involving modifications in the ACL. In addition, in bacteria and yeast, most essential genes, or genes whose deletion leads to marked growth phenotypes, are involved in the introduction of modifications at positions 32, 34 or 37 in the ACL. These positions and their respective modifications have therefore been the focus of many of the studies on tRNA modifications and their associated functions. Overall, modifications of the tRNA core may have been neglected in comparison. In recent years, however, a number of studies have sought to characterize and understand the role of some of these modifications (a few examples are shown in Figure 2).