There are currently many computed tomographic units with differing performance capabilities, resulting in confusion over protocols for contrast medium usage. Additionally, the development of the safer but more expensive low osmolar contrast agents has increased interest in evolving clearer indications for contrast medium usage in C.T. scanning. Rationales for the use of intravenous agents in C.T. scanning are proposed. These include the labelling of normal structures for anatomic clarification, the assessment of perfusion, the characterisation of a specific lesion, CT Angiography, demonstration of defects in the blood brain barrier and the phenomenon of neovascularity. The logical use of contrast agents should involve the deliberate invocation of one or more of these mechanisms coupled with the appropriate technique of administration.