Threonine contributes to the solubility and reactivity of proteins by its hydroxy group as well as to the formation and stability of the hydrophobic core of proteins by its methyl group. One may assume that the use of this bifunctional and simply structured amino acid was established early in evolution. Whereas the catalytic pathway of threonine activation and transfer into protein does not deviate essentially from those catalyzed by other aminoacyl-tRNA synthetases, the enzyme specific for threonine exhibits several interesting individual properties: its biosynthesis is regulated by feedback mechanisms, it can be selectively inhibited (out of twenty aminoacyl-tRNA synthetases) by the antibiotic borrelidin, and it can be a target for autoantibodies, thus being involved in the course of autoimmune diseases. The enzyme has been isolated from more than ten organisms showing a dimeric nature and molecular masses between 110 and 220 kDa. Additionally, in several of these cases, the gene of threonyl-tRNA synthetase has been localized, cloned and sequenced, exhibiting proteins of 400 to 800 amino acids chain length. More interesting facts can be expected from future research ranging from chemistry and molecular biology to medicine, e.g. by elucidation of the three dimensional structures of threonyl-tRNA synthetases and of their antigenic epitopes, possibly followed by therapeutic use of less antigenic mutant proteins.