Mosses (division Bryophyta) are characterized by the dominance of haploid, poikilohydric gametophytes, and relatively persistent sporophytes that are dependent on the gametophyte generation. Because they are poikilohydric, tend to be desiccation tolerant, and have primarily ectohydric water uptake mechanisms, the ecological requirements of mosses tend to differ from those of vascular plants. We review recent research on phylogenetic relationships and morphological and ecological diversity among mosses. Ancestral character state reconstructions illustrate the evolution of habitat preferences and the relationships between these and morphological variation. These reconstructions reveal the convergent evolution of both epiphytic and aquatic habitat preferences, as well as several reversals from epiphytic to other terrestrial habitats. Morphological character states connected to ectohydry, such as lack of water-conducting stem central strands, may be more prone to adaption driven by environmental conditions, while connections between endohydry and environmental conditions remain ambiguous and require further study. The distribution of the most elaborate endohydric water-conducting structures may be phylogenetically determined rather than resulting from adaptation to habitats. Among the early diverging lineages in the Bryophyta, shifts to cladocarpy may be connected to epiphytic lifestyles. We discuss ecological drivers of aspects of plant architecture including acrocarpous, pleurocarpous and cladocarpous perichaetial positions, and sporophytic reductions, the latter being common in dry, frequently disturbed terrestrial environments and in epiphytic habitats, but also occasionally found in aquatic habitats.