The authors state that the chapters contained in the book were put together in an effort to enhance the application of rational and systematic methods to police investigations. Psychologists have important contributions to make to criminal investigations, but they must be cautious that their work is accurate and based on sound theory and scientific methods. While assistance from psychologists was once shunned by investigating officers, today’s climate is more welcoming of outside expertise, if still somewhat skeptical. Before psychologists offer their expertise in criminal cases, there are a number of issues that should be clarified, including the specific objectives of the psychological assessment and expectations of the final report. An “investigative credo” is offered that will assist any psychologist in carrying out and preparing a psychological assessment for law enforcement purposes. The “investigative credo” entails a list of 27 questions that psychologists must ask themselves while preparing psychological assessments. Questions include inquiries about the time to investigate and prepare a report, about the type of evidence available for review, and about the provisional hypotheses of the case. Overall, psychologists have much to offer law enforcement and criminal justice systems; in order to offer the best product, psychologists must rely on professional ethics and standardized methods. Notes, references