Carbon allocation is a key process in ecosystems: it is coupled with plant growth, fuels metabolism and plays a crucial role for carbon sequestration in standing biomass and soil organic matter. While the importance of carbon allocation for plant and ecosystem functioning and the carbon balance is widely recognized, we still lack a comprehensive understanding of the underlying mechanisms, responses to global changes and wider biogeochemical implications. Open questions include: 1) what drives carbon allocation in plants and ecosystems?; 2) what is the fate of newly assimilated carbon?; 3) what determines the allocation of nonstructural carbon to growth, metabolism and storage?, 4) how does carbon allocation affect nutrient and water relations in plant and ecosystems?; and 5) how do allocation patterns change under changing environmental conditions and what are the consequences for biogeochemical cycles? This session invites contributions from observational, experimental and modelling studies.