The circumscription of the family Agaricaceae, its genera and their characters are investigated. Analyses of the ITS and LSU sequence data, separately and in combination result in exclusions and reorganization. Notholepiota areolata, a secotioid species from New Zealand, Cystoagaricus with brown, irregular-knobby spores, and Leucopholiota with gelatinized hymenophoral trama and amyloid spores, fall outside the family Agaricaceae. In the reorganization, long accepted tribe and genus definitions are amended, viz.: (1) Macrolepiota and Chlorophyllum separately are monophyletic, but not together; (2) Leucoagaricus and Leucocoprinus together form one large monophyletic clade, that also includes polyphyletic Sericeomyces; (3) Lepiota, Cystolepiota and Melanophyllum together form a monophyletic clade; (4) Secotiation has happened within several clades (e.g. Agaricus and Chlorophyllum), though not in the Leucoagaricus/Leucocoprinus clade, nor in the lineage of Macrolepiota proper; gasteroid and most secotioid species in the course of adaptation acquire coloured spores; and (5) the monovelangiocarpic Chamaemyces fracidus appears to be basal to the family and provides clues to the morphology of the ancestral Agaricaceae. Trama and developmental characters characterize the different clades, whereas spore characters play a minor role.