The 5´-terminus of eukaryotic messenger RNA (mRNA) molecules contains an unique structure, viz. a dinucleoside 5´,5´-triphosphate moiety where the terminal nucleoside is 7-methylguanosine. This 5´-cap structure serves as a site of recognition for numerous enzymes involved in splicing, transport and translation of mRNA, and protects mRNA against intracellular exonucleases. In addition, viral RNA polymerases use capped mRNA sequences of the host cell as primers for viral RNA synthesis. Understanding of molecular mechanism of all these processes requires detailed information on the chemical properties of the cap structure, including capability to prepare conjugates and structural analogs of the cap for research tools. This review tends to summarize the present knowledge on various aspects of the chemistry of the cap structure, including chemical stability and mechanisms of breakdown, protolytic and complexing equilibria, stacking interactions and synthetic methodology.