Prosecutors handling impaired-driving cases may decide to use standardized field sobriety test officers, drug recognition evaluators/experts, toxicologists, crash reconstructionists, optometrists, and other medical personnel as witnesses to prove some fact or element of the case against the defendant. This publication provides guidance as to what it means to be a witness, what can be expected when serving as a witness, and the importance of the witness role. It also provides an overview of the complexities of the criminal justice system to help explain why a witness may be called to testify more than once, only to be asked the same questions on the same subject for the same case. This guide is divided into four parts. Part I provides an introduction and overview. Part II discusses understanding criminal cases by providing information on the role of law enforcement officers and expert witnesses in impaired-driving cases, constitutional principles, and stages of a criminal case. Part III discusses law enforcement testimony by describing standardized field sobriety test officers, and drug evaluation and classification testimony. Part IV discusses scientific experts (toxicologists, crash reconstructionists, optometrists, and the medical examiner/forensic pathologist) as well as cross-examinations.