The dynamics of the age class structure stands at the center of modeling long-run forestry scenarios. This insight has been applied to the construction of the Forest Development and Carbon Budget Simulation Model (ForCaBSiM), a model which is used for the study of several interrelated questions: the development of timber stocks and the potential level of sustainable harvests, the stocks and fluxes of tree carbon in managed forests, the economy-wide effects of management practices on the value of forest lands and timber stocks. The combined study of these issues allows to assess development scenarios with regard to the productive potential of forestry, the carbon cycle, and forest values. At present, the model is adapted to German data, but it is designed for use with other data sets as well. This paper provides a description of core mechanisms in ForCaBSiM. On this background, the choice and impact of crucial assumptions is examined. Illustrative results are used to demonstrate the use of the model. The paper focuses on the impact of varying rotation ages and the tree species composition. Particular attention is given to the concept of steady states.