Prevailing liberal rules of evidence permit qualified medical and scientific experts to offer opinions designed to help courts decide issues to which their expertise relates. The opinions can be based on direct examinations, review of data assembled by others and data or inferences of a type relied on by other experts in the field. Application of these rules is illustrated through analysis of expert testimony in litigation involving a neurologic syndrome allegedly caused by an immunization and in a case involving controversy over the extent and outcome of major brain injury. Concerns about misuse of expert medical and scientific testimony in litigation are addressed. The article closes with a consideration of approaches designed to improve the reliability of expert testimony.