This paper provides a theoretical explanation for the widely debatedempirical finding of “Environmental Kuznets Curves”, i.e., U-shaped relationships between per-capita income and indicators of environmentalquality. We present a household-production model in which the degradationof environmental quality is a by-product of household activities. Householdscan not directly purchase environmental quality, but can reduce degradation by substituting more expensive cleaner inputs to production for less costlydirty inputs. If environmental quality is a normal good, one expectssubstitution towards the less polluting inputs, so that increases in incomewill increase the quality of the environment. It is shown that this onlyholds for middle income households. Poorer households spend all income on dirty inputs. When they buy more, as income rises, the pollution also rises.they do not want to substitute, as this would reduce consumption ofnon-environmental services for environmental amenities that are alreadyabundant. Thus, as income rises from low to middle levels, a U shape can result. Yet an N shape might eventually result, as richer households spend all income on clean inputs. Further substitution possibilities are exhausted.Thus as income rises again pollution rises and environmental quality falls.