This chapter begins with a brief overview of the origin and nature of the twin method. Comprehensive descriptions of the rich array of twin designs—some conventional, others unconventional—follow, each accompanied by illustrative research drawn from the extant psychological literature. The next focus is on research addressing the unique dynamics and complexities of twin relationships. The final portion of this chapter presents highlights, applications, and future directions of behavioral genetic studies. Examples include findings from new studies of novel phenotypes, the relevance of MZ twins’ behavioral resemblance and emotional closeness in various courtroom cases, and the importance of forging closer connections between behavioral genetics and evolutionary psychology.