Computed tomography's exceptional contrast sensitivity, which can show local changes in absorption coefficient of 0.5% of the absorption coefficient of water anywhere in the cross-section, was truly revolutionary compared to conventional radiography's ability to distinguish perhaps a 2% change in absorption in the total transmission path through the object. This contrast sensitivity combined with 3-D localization has given CT a preeminent position in neuroradiology and has lead to the rapid development of CT for applications in the rest of the body. To date, almost all of these applications have involved static or steady state scans. One current extension of CT is its use in transient studies, for example to follow a bolus of radiopaque in its passage through an organ such as the heart. This technique has been used to demonstrate regional blood flow abnormalities in animal experiments and coronary bypass patency in patients. The performance of CT scanners is well understood and major performance characteristics can be quantitatively calculated or predicted. These include both high contrast resolution and low contrast detectability, dose, and even some types of artifacts. For conventional static scanning the most important questions are not technical, but relate to the clinical utility and cost effectiveness of various possible designs. For designs aimed at transient studies, x-ray source brightness is probably the most significant technical limitation. Most CT scanners are close to being source brightness limited, with room for only modest improvement before system trade-offs are required. The resulting system constraints combined with CT's high visibility in society's effort to control health care costs suggest that future CT developments are likely to evolve with considerably clinical interaction and guidance.