A review and synthesis of the available literature is presented concerning the structure of the moss gametophyte as it is adapted to the environment and the survival of the moss. New observations are made, when appropriate, and a number of new hypotheses are given. Spore germination patterns, protonemal structure, life span and methods of vegetative propagation are all related to the nature of the habitat. Multicellular spores and swollen protonemal cells are strongly correlated to environments subject to periods of desiccation. Highly specialized protonemata are found in deeply shaded environments; "typical" protonema, with strongly elongate cells, tends to characterize taxa of mesic sites or of terrestrial habitats where moisture conditions are favourable for at least part of most days in the growing season. The gametophore exhibits several growth forms that are environmentally restricted or, in some cases, are apparently moulded by the environment, thus the phenotype is variable. Gemma production in many mosses is vigorous while the gametophore remains vegetative. When sex organs are produced, gemma production tends to be reduced or to cease. Water uptake in the gametophore involves ectohydric and endohydric systems. In ectohydric systems, water movement is influenced by leaf shape, arrangement, orientation and detailed anatomy, to branch arrangement, nature of cortical cells, presence of rhizoids or paraphyllia. Surface ornamentation of leaves, stems and rhizoids are important in habitat specificity. In endohydric mosses, hydrome and leptome systems are sometimes significant in habitat specificity, especialy in the subclass Polytrichidae. Detailed experimental research concerning adaptive significance of moss gametophyte morphology remains scanty, thus most generalizations are tentative.