In contemporary organisms the aminoacylation of tRNAs is performed exclusively by protein aminoacyl-tRNA synthetases. However, in vitro selection experiments have identified RNA enzymes that exhibit the necessary characteristics to charge tRNA molecules with acyl groups in a way that is compatible with ribosomal translation, suggesting that such ribozymes may have fulfilled this function prior to the evolution of proteinaceous life. The current chapter provides a review of the history, structure, and function of these RNA aminoacyl synthetases, and discusses their practical application to "genetic reprogramming" and other biotechnologies.