This Working Paper is part of a study aiming to relate long term environmental change, population growth, production and technological change, and to identify the policies and institutions which are conducive to sustainable development. The first stage, published in these Working Papers, is to measure and assess as precisely as the evidence allows the changes that have occurred in the study area, the semi-arid Machakos District, Kenya, over a period of six decades. Degradation of its natural resources was evoking justifiable concern in the 1930s and 1940s. It now appears to be in a more sustainable state, despite a five-fold increase in population. The long-term perspective is required because temporary factors, such as a run of poor rainfall years, can confuse analysis of change. The study is developing a methodology for incorporating historical, physical, social and economic data in an integrated assessment.