This paper examines the relaionship between “population pressure” and socioeconomic complexity among hunter-gatherers. Population pressure is defined as the ratio between population density and the density of available resources. Socioeconomic complexity is measured by means of several correlated variables: storage-dependence, sedentism, social inequality, and use of a medium of exchange. Correlations between these variables are calculated from an ethnographic sample of 94 hunter-gatherer groups. The correlations between population pressure and socioeconomic complexity are found to be extremely high. Two major types of hunter-gatherers exist which are distinguished by a number of variables and may be termed “simple” and “complex.” Transitional groups between these two types are quite rare. It is also noted that population pressure does not arise in continental climates where famine mortality is common because of high-amplitude changes in productivity from year to year. It is argued that population pressure is a necessary and sufficient condition for and the efficient cause of socioeconomic complexity. The widespread disavowal by archaeologists of population pressure as a possible explanation for the prehistoric development of complex hunter-gatherers has no basis in ethnographic fact.