The connection between economic growth and the quality of the environment is a popular subject. Since the second half of the 1980s, there has been an increasingly frequent use of the term "sustainable development," to refer to the preoccupation that the production of goods and services may decrease standards of living. It is natural to analyze this question from the point of view of economic models, which may be helpful in at least identifying the main factors behind such preoccupations and perhaps in suggesting policy measures. Indeed, models are useful to discuss some relevant factors, like...