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The tundra-taiga interface, or forest-tundra (lesotundra in Russian, and toundra forestière in French), corresponds to the subarctic zone where the closed boreal forest gradually becomes less dense and progressively breaks down into tree islands towards the treeless, Arctic tundra (1). The interface does not form a sharp limit of tree growth in the landscape but is instead a north-south transition zone, which often can extend several hundred kilometers from the continuous forest limit, particularly in Canada and Russia (2–6). The northernmost part of the tundrataiga interface coincides with the Arctic treeline, which is controlled by cold temperatures, precipitation deficits, strong winter winds, lack of snow protection, as well as past and present influence of human activity particularly in Fennoscandia and Russia. Indeed, the total number of climatic and other forcing factors restricting tree growth and regeneration in the forest-tundra is large and regionally variable (5, 6). The terminology referring to the tundra-taiga ecotone is relatively well established (7, 8) with forest limit and Arctic treeline as the most important boundaries. The treeline, with trees having a minimum height of 2.5 m, forms the northernmost limit of tree growth. The forest limit corresponds to the northernmost forest, i.e., a stand of several tens of trees with most stems developing vertically (1, 5, 6, 8). The tree species limit extends north of the Arctic treeline in tree species able to grow as shrubs in the circumpolar tundra. The overall latitudinal temperature gradient change is rather minor when one moves north in flat areas, whereas the change is stronger with elevation. As a result, the vegetation belts in mountainous areas are often sharper and narrower than in lowlands. Treeline changes of the northern edge of the forest-tundra are referred to as Arctic treeline shifts while those within the forest-tundra are called subarctic treeline shifts (1). In contrast to alpine treelines, subarctic treelines are not necessarily controlled by elevation, but instead describe the boundary between tree and treeless communities on well-drained flat and rolling terrain. Therefore, the changing position of subarctic treelines denotes a change in abundance of forest-tundra trees, whereas a changing position of the Arctic treeline corresponds to the latitudinal displacement of the forest-tundra boundary. A southward shift of the boundary indicates death of trees in the northernmost tree outposts. A retreat of the subarctic treeline is due to trees being unable to regenerate at a site and indicates a decreasing abundance of forest-tundra trees. The tundra-taiga across the circumboreal zone is inhabited by several tree species that experienced major geographical shifts caused by interrelated forcing factors since the last glacial maximum at 20 ka (thousands of years ago, i.e., cal. years BP) (9). The reconstruction of past and recent positions of the Arctic treeline has been a much debated topic during the last three decades. The topic is important for its relevance in the study of the projected magnitude of environmental changes associated with greenhouse warming. In this paper, we review the evidence of treeline dynamics across the circumboreal zone, from the last glacial maximum (i.e. Wisconsinan in North America and Weichselian in Eurasia) to present, and comment on the likely causal factors.