More than 100 different types of chemical modifications are found in cellular RNAs, including ribosomal RNA (rRNA), transfer RNA (tRNA), messenger RNA (mRNA), long non-coding RNA (lncRNA) and small nuclear RNA (snRNA). Internal modifications of mRNA were first observed in the 1970s, but, until recently, the role of these mRNA modifications has been a largely neglected field. A long-standing view was that mRNA modifications were static and unalterable, having a minor role in fine-tuning the structure and function of mRNAs. However, recent exciting discoveries now suggest that certain mRNA modifications are dynamic and, in some cases, reversible. Therefore they may have critical regulatory roles in gene expression, analogous to those which dynamically regulate DNA and protein modifications. As such, understanding the scope and mechanisms of these dynamic mRNA modifications represents an emerging layer of gene regulation at the RNA level, termed epitranscriptomics or RNA epigenetics.