One of the many unique features of Wave III of the Add Health Study was the collection of biological samples. These biological samples permitted the identification of individuals with sexually transmitted infections [STI] (including HIV), and genotype ascertainment for pairs of full-siblings or twins who resided in the same households. The STI testing allows for analyses of individual, household, family, and environmental risk factors for laboratory-confirmed sexually transmitted infections (versus self-report), and the genetic sample facilitates analyses that differentiate between parental, social, and genetic influence, as well as the extent to which genetic differences in neurotransmitter function are associated with a wide range of behaviors. The inclusion of these biomarker data requires special considerations in the analysis of Wave III Add Health data. Thus, the purpose of this monograph is to outline relevant procedures, design, and sampling schemes used in the collection of biomarker data, and to serve as a user’s guide for its analysis and interpretation. The monograph is intended to supplement existing descriptions of the Add Health study, rather than to replace them. Therefore, please refer to the web pages describing the Add Health Study design for more extensive detail on the study (www.cpc.unc.edu/addhealth) and the sampling weights necessary to work with the data (www.cpc.unc.edu/addhealth/codebooks/wave3). Issues that require special consideration include sample design (e.g., who was selected for each type of biomarker test), specimen collection, laboratory methods, and laboratory test performance. Each of these themes is described in separate chapters to this monograph, but should be viewed as complementary to each other.