Flat-panel X-ray detectors used in medical imaging applications present a challenge to failure analysts due to the scale of the products, the newness of the technology, and the relatively low production rates compared to ICs. This article explains how existing tools are being adapted to accommodate the size of these detectors and the exotic materials from which they are made. It discusses the types of defects that can occur and how they affect critical detector characteristics. It describes the basic approach for defect localization and physical analysis and presents examples of defects in different areas of a flat-panel X-ray detector.