This chapter provides an introduction to structural equation modeling (SEM), a statistical technique that allows scientists and researchers to quantify and test scientific theories. As an example, a model from behavioral genetics is examined, in which genetic and environmental influences on a trait are determined. The many procedures and considerations involved in SEM are outlined and described, including defining and specifying a model diagrammatically and algebraically, determining the identification status of the model, estimating the model parameters, assessing the fit of the model to the data, and respecifying the model to achieve a better fit to the data. Since behavioral genetic models typically require family members of differing genetic relatedness, multisample SEM is introduced. All of the steps involved in evaluating the behavioral genetic model are accomplished with the assistance of LISREL, a popular software program used in SEM.