The tundra-taiga interface is a dominant vegetation boundary that is related to climate and has an importance at a global level for its contribution to land atmosphere interactions, biodiversity and land use. However, our understanding of the precise location, dynamics and characteristics of the boundary, and its environmental and biotic drivers at a circumpolar level is poor. Our understanding has been constrained for various reasons, perhaps including a quest by researchers to denote 2- or even 3-dimensional tree distribution limits to a single line on a map. Current rapid sociological and environmental changes in the north necessitate better definitions to be made of characteristics associated with the tundra-taiga interface so that changes can be monitored and identified, and implications of these changes can be assessed. This concept paper introduces some of the complexities of adequately defining the boundary and suggests characteristics and processes that could focus future research at a collaborative, circumpolar level to create baseline data and to monitor and predict changes in the boundary zone.