This essay focuses both on theoretical issues and empirical evidence concerning the inevitability of slavery as an economic stage over the course of development. Different types of slavery are described and a number of common hypotheses about the determinants of slave societies and/or slavery are discussed. Several new hypotheses are also proposed which focus upon family and social-structural variables and also upon political factors. The major hypotheses are tested, using a worldwide sample of 60 primitive and peasant societies arranged according to level of economic development. The various older theories are found to explain poorly the presence of slavery while the data give some support to the newly proposed causal relationships.