This book examines variation in the economic attainments of immigrants to the United States from Latin America, relying primarily on data drawn from the 1990 census. The purpose is to explain as accurately as possible why this variation occurs. Typically, researchers have chosen between two competing explanations. One, human capital theory, contends that variation in economic attainment is a product of different characteristics of individuals. The other, social capital theory, contends that variation in economic attainment is a product of differences in characteristics of the societies from which the workers come. The question is whether we must choose one over the other. Given that both theories have empirical support, the challenge is to build an appropriate methodology that allows us to partition the variation in economic attainments, identifying how much is explained by individual and how much by group characteristics.