N(6)-methyladenosine (m(6)A) is a modified base that has long been known to be present in non-coding RNAs, ribosomal RNA, polyadenylated RNA and at least one mammalian mRNA. However, our understanding of the prevalence of this modification has been fundamentally redefined by transcriptome-wide m(6)A mapping studies, which have shown that m(6)A is present in a large subset of the transcriptome in specific regions of mRNA. This suggests that mRNA may undergo post-transcriptional methylation to regulate its fate and function, which is analogous to methyl modifications in DNA. Thus, the pattern of methylation constitutes an mRNA 'epitranscriptome'. The identification of adenosine methyltransferases ('writers'), m(6)A demethylating enzymes ('erasers') and m(6)A-binding proteins ('readers') is helping to define cellular pathways for the post-transcriptional regulation of mRNAs.