'Biological progress' in forestry has been fast in the sense that volume growth functions of particular species have improved considerably over time. The site quality classification index used in the 1930s in Sweden shows that the present yield is 30% above the yield considered to be the maximum at that time. Much of the actual progress is due to improved silviculture or to a poor site quality classification index, but some is certainly due to better genetical material. This chapter explores how genetical or simply biotechnological progress affects the socially optimal rotation period and the value of forest land. It considers the unrealistic case of a regularly exponentially improving biotechnology. It is possible to model changes in biotechnology by introducing a parameter into the growth function. In addition, it is necessary to specify more or less explicitly how a change of the parameter changes the properties of the growth function.