This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book presents some general themes dealing with DNA testing in the various national criminal justice systems. It raises recurrent concerns about consent of the individual to the test, protection of privacy of genetic information, dignity of the individual and the overriding requirement for fairness in the criminal investigation and trial process. The book indicates that legal issues relating to the interpretation of test results, the accuracy and reliability of test results and the admissibility of test results in evidence remain within the general law of evidence. It presents a broadly common position that genetic testing may occur at any time during the investigative period, subject to prescribed procedures and authorisation by a police officer or, in other cases, by a judicial officer.