This chapter presents an overview of key concepts discussed in this book. The book examines the three earlier classical reared-apart twin studies. It focuses on twin research used to assess the role of genetic influences on behavioral differences. The main research methods used by human behavioral genetics are family, twin, and adoption studies, with twin studies playing a predominant role. Family studies assess the behavioral resemblance or diagnostic status of biological relatives who share a common environment as well as common genes, and may include other biological relatives in different branches of the family. The book discusses several important issues related to psychometrics. It looks at some basic yet controversial concepts used in twin research and behavioral genetics in general, which include IQ testing, personality, and heritability. The book examines some basic assumptions in psychiatry that have been the subject of criticism for many years, such as the reliability and validity of its diagnoses.