Genes associated with behavioral dimensions and disorders are beginning to be identified. Although it is difficult and expensive to find genes associated with behavior, it is relatively easy and inexpensive to use genes that have already been identified. We describe how genes are found, but the main goal of this article is to outline what developmentalists can do with genes once they are found and, hence, to encourage the use of DNA markers in developmental research. We suggest that genes can be used to answer questions about developmental continuities, about psychopathological patterns, and about environmental risk mechanisms. Developmental questions include the causal mechanisms involved in heterotypic continuity. Questions on psychopathological patterns address heterogeneity (Do gene-behavior associations apply to disorders or to separate components representing risk or protective factors?), comorbidity (Are gene-behavior associations diagnosis-specific?), and the links between normality and disorder (Does a gene-behavior association for a disorder extend to related dimensions of normal variation and vice versa?). Questions about environmental risk mechanisms are informed by study of gene-environment interaction (Are individuals who are at genetic risk more sensitive to specific psychosocial risks?) and gene-environment correlation (Are individuals who are at genetic risk more likely to be exposed to psychosocial risk?).