A common aim of the poor has been described as a secure and decent livelihood (Chambers 1983). In the rural areas of poorer countries, most people strive to achieve this through farming. When the farming system is under the control of the rural people themselves, it is reasonable to assume that in many cases its primary objective will be to ensure the long-term maintenance of that livelihood. Most economic analyses of such systems explicitly or implicitly value them in terms of some measure of output. These productivity measures may represent the utility of the system to some external but interested party, such as a government, but do not necessarily reflect the utility of the system to the producers themselves. Households are constrained by the need to raise some basic level of food and income - enough to meet their subsistence needs - every year. In this paper I describe a model, developed by Mace and Houston, which illustrates how, in unpredictable environments, maximising the short-term mean level of output and maximising the long-term viability of the households producing that output are not necessarily the same thing. The most rational option for the household is to maximise the latter, which may involve increased diversification and lower rates of offtake than the former. Suggested "improvements" to management practices that increase productivity may not be sustainable because they reduce household viability. This is a model of a livestock production system which evaluates herding strategies, and the impact of external events upon them specifically in terms of their effect on the long-term maintenance of a basic standard of living for the pastoralists.