The QDU supports Federal, State, local, and international law enforcement agencies by providing advanced technical and forensic support with: (1) examinations, reports, and testimony; (2) technical support of field investigations; (3) training provided to FBI and other Federal and State examiners; and (4) research and evaluation of newly developed technology. The author describes the various types of examinations and technical assistance available from the QDU and then presents case examples that show how the forensic or technical support significantly aided a case. Two main types of examinations are offered by the QDU: physical examinations and comparison examinations. Physical examinations generally involve a careful scrutiny of surfaces and other items. The most common surface submitted for QDU examination is paper. The case example of the murder of Heather Lee Sims and the subsequent conviction of her mother for first degree murder are offered to illustrate how forensic comparisons of plastic bags helped to solve the case. Comparison examinations are conducted on handwriting, hand printing, typewriting, and typewriter components such as ribbons and elements. Comparison examinations are also used for other forms of evidence such as facsimiles, photocopies, and printing processes. The author describes handwriting comparison in some depth and then presents the case of the kidnapping and subsequent death of 1-month-old Peter Weinberger in 1956, which handwriting analysis helped to solve. Typewriter comparisons are also described followed by a discussion of how the infamous Unabomber was foiled through typewriter analysis. The article also presents the reference and standards files maintained by the QDU, outlines protocols for typical cases, and explains how FBI QDU examiners are trained. Figures, references