Household C/P ratios, as Chayanov argued, may induce bottlenecks in food availability relative to consumers once every generation or so. Environmental and agricultural seasonality, on the other hand, often induces once-a-year (or more frequent) bottlenecks in absolute food availability, to which the household must adapt every year or suffer the consequences. The causes and magnitude of seasonal swings in the food supply differ from place to place, but there is no farming environment in the world that is absolutely aseasonal. In many places, moreover, the seasonal swings can be large enough to threaten the household demographic enterprise. It seems appropriate, therefore, to include seasonality as a critical dimension of household demographic adjustment, as I do in this chapter. But before we examine the agronomic and demographic impact of seasonal changes, I want to take a brief detour to review global patterns of environmental seasonality and their causes, as well as to note a couple of stubborn problems inherent in the analysis of seasonality’s effects.