The palaeoclimatic importance of fossil arctic plants has been realised since the discovery of sediments containing remains of arctic plants, particularly in lowland Britain, Denmark, and Sweden in the late nineteenth and early twentieth centuries, very far from where arctic plants live today. Arctic and alpine plants characterise treeless habitats above or beyond the tree line. They may be adapted to harsh conditions and intolerant of lowland temperatures or be confined to these habitats through competition from larger, more competitive plants. Much more is known of the history of arctic plants than of alpine plants. In Late-Quaternary glacial periods, arctic plants spread into cold (and often arid) unglaciated areas in central Europe and Eurasia, Siberia, Beringia, and North America. Their effective propagule dispersal allowed them to follow suitable habitats at the fluctuating edges of the ice sheets and beyond. During deglacial warming many species expanded and flourished on freshly deglaciated terrain before larger plants immigrated. In north-west Europe many arctic-alpines were locally exterminated during the warm Allerød-Bølling Interstadial but spread again in the cold Younger Dryas Stadial. Rapid Holocene warming forced species northwards or upwards as trees invaded the landscape. Plants that are tolerant of warm temperatures but intolerant of competition persisted in lowland refugia in open habitats such as sea-shores and cliffs. As global climate warming proceeds today and human impact increases, the area available to arctic and alpine plants will continue to decrease. Species particularly in danger are those with small populations and those that are intolerant of warm temperatures.