This list provies a guide to the most important operas, as determined by their presence on a majority of compiled lists of significant operas: see the "Lists Consulted" section for full details. The operas listed cover all important genres, and include all operas regularly performed today, from seventeenth-century works by Monteverdi, Cavalli, and Purcell to late twentieth-century operas by Messiaen, Berio, Glass, Adams, Birtwistle, and Judith Weir. The brief accompanying notes offer an explanation as to why each opera has been considered important. For an introduction to operatic history, see Opera. The organisation of the list is by year of first performance, or, if this was long after the composer's death, approximate date of composition.