A shoot: root partitioning model is presented, which is a development of previous approaches in the area. The model incorporates as a physiologically reasonable apparent ‘goal’ for the plant, the assumption that the partitioning of growth between the shoot and root maximizes the plant specific growth rate in balanced exponential growth. The analysis is concerned principally with plant growth being a function of carbon and nitrogen only, although it is indicated how other nutrients, or growth factors, may be incorporated. Plant growth is driven by the environmental conditions, and partitioning is defined entirely in terms of the shoot: root ratio and carbon and nitrogen status of the plant. In its basic form the model requires the definition of a single plant growth parameter, along with the shoot and root specific activities and structural composition.