Forensic psychology is the application of psychological research and practice to both the criminal and civil justice systems. It is a specialty of psychology with endless fascination for both the public and the professional. In this chapter, we consider the knowledge base, providing an overview of practice guidelines, expert witness rules and practice, and legal decisions that affect the practice of clinical forensic psychology. We then turn to a review of major activities and roles of forensic psychologists and the application to forensic assessment and treatment in both the criminal and civil justice systems. We conclude with key achievements of forensic psychology as a specialty and its future directions.