Although everyone now understands that both genes and environment are important, behavioral genetics is still a long way from being a central part of training in either psychiatry or psychology research. I continue to be struck by the fact that one of the most common questions I am asked about my own twin research is still, "Are they all identical?" To the behavioral genetics researcher, of course, this conveys a lack of basic understanding of the method. Yet with recent advances in molecular genetics and genomics, understanding behavioral genetics and conducting research that is genetically informative will surely become increasingly important. With these concerns in mind, this book serves as an excellent introduction for newcomers to behavioral genetics, while it also addresses sophisticated and provocative issues that will be both interesting and useful to more seasoned researchers.