The doctoral degree is the entry degree for an expert psychological witness. Predoctoral and postdoctoral training is also important, as are experience, scholarship, aptitude in the courtroom, reputation, and history as an expert witness. The professional opinion offered in the courtroom is based both upon professional data and upon professional conclusions. Firsthand data are from clinical observations and formal and informal test results. Secondhand data that are highly acceptable include the personal history, the public record, and previous clinical reports. Effective expert witnesses are prepared, exact, brief, coherent, responsive, and controlled. Common pitfalls to avoid are poor scholarship, poor clinical work, poor preparation, poor control, poor grasp of the issues, and poor presentation. Poor preparation is often observed in the following areas: poor checking, poor layout, poor review, poor rehearsal, poor timing, poor exhibits, and poor immersion. Flawed presentations can be inaudible, incoherent, too sophisticated, too jargony, too fast, too weak in examples, too lacking in repeatable evidence, or too vague. 40 references.