The practice of clinical diagnostic radiology has been made possible by advances not only in diagnostic equipment and investigative techniques, but also in the contrast media that permit visualization of the details of the internal structure or organs that would not otherwise be demonstrable. The remarkably high tolerance of modern contrast media has been achieved through successive developments in chemical pharmacological technology. A single dose of X-ray contrast medium commonly contains upwards of 2000 times as much iodine as in the total physiological body content, and yet it is cleared from the system rapidly and naturally, usually with no adverse effects at all. The choice of contrast medium has always been a matter of debate, but is ultimately the responsibility of the radiologist. In order to be able to make a rational decision as to the selection of contrast media, it is necessary to have some understanding of the physical and physiological principles involved. The objective is to provide a background for non-specialists on this complicated specialist subject.