N⁶-methyladenosine (m⁶A) is the most prevalent and internal modification that occurs in the messenger RNAs (mRNA) of most eukaryotes, although its functional relevance remained a mystery for decades. This modification is installed by the m⁶A methylation “writers” and can be reversed by demethylases that serve as “erasers.” In this review, we mainly summarize recent progress in the study of the m⁶A mRNA methylation machineries across eukaryotes and discuss their newly uncovered biological functions. The broad roles of m⁶A in regulating cell fates and embryonic development highlight the existence of another layer of epigenetic regulation at the RNA level, where mRNA is subjected to chemical modifications that affect protein expression.