Many medical procedures are performed under fluoroscopy. Relatively large metallic devices are readily visible without much regard for the materials of construction. However, small devices like catheters often require radiopaque ‘marking’ with components made from highly radiopaque materials. This paper will review the theory of x-ray attenuation, and it will review the radiopacity of metals applicable to medical devices. The effect of component geometry on visibility will be discussed, and various approaches to radiopaque marking will be described. Finally, some properties indicative of MRI compatibility will be reviewed for the highly radiopaque materials.