A simple approach was used to evaluate the potential human population that the pre-industrial Icelandic environment could sustain. A model was constructed that simulated the population size according to potential biological production available for livestock. Biological production was determined by the extent of the total poten-tial vegetation cover based on the Degree-day concept. Fluctuations in the mean an-nual temperature cause changes in the potential vegetation cover and as a conse-quence change the biological production sustaining livestock and ultimately human population. The simulation's results indicate that the potential population that the environment could sustain during the pre-industrial period fluctuated around 40-80 thousand. The results further indicate that the severe land degradation experienced after the settlement period had a marginal impact on the population size. The pre-historical population did however overshoot the natural sustainability on few occa-sions.