Understanding of male dwarfism in mosses is reviewed with special reference to the genus Dicranum. Dwarf males occur in about 20 of Dicranum species. Most species seem to be obligately nannadrous; in two species (D. bonjeanii, D. scoparium) male plant size may be normal or dwarfed. Variation in male plant size in Dicranum seems to be environmentally controlled. Male dwarfism in mosses is induced by genetic or environmental factors although the mechanisms leading to male dwarfism are poorly understood. Evidence suggests that male dwarfism increases reproductive success in dioicous species but many questions remain unanswered.