The intense effort in the past 5 to 10 years to develop on-line image verification (OLIV) systems is made partly in anticipation of the increased verification demands of complex 3-dimensional (3D) conformal radiation therapy, and partly to improve on the current practice of weekly treatment verification. These systems allow convenient acquisition of daily portal images or many images during one treatment session. Systems based on fluoroscopic and scanning approaches can now be purchased from various vendors and are being evaluated at several radiation therapy centers. The performances of these systems vary, but all appear to be adequate for clinical use. At the Mallinckrodt Institute of Radiology, we have been conducting clinical on-line imaging for the past 3 years using a fiber-optic fluoroscopic system. Daily operation of the system requires coordinated participation of the physics and radiation therapy technology staff. The major consideration is in the management and evaluation of the large amount of verification images about each individual patient. At present, it is not clear whether the technology would be cost-effective in the clinical setting. However, it is clear that OLIV provides a powerful tool in enhancing our knowledge of treatment variation. The information will be invaluable in the development of improved treatment techniques. It is also likely that these systems will play a important part in the verification of 3D conformal radiation therapy.