Progress in radiotherapy is guided by the need to realize improved dose distributions, i.e. the ability to reduce the treatment volume toward the target volume and still ensuring coverage of that target volume in all dimensions. Poor ability to control the tumour's location limits the accuracy with which radiation can be delivered to tumour-bearing tissue. Image-guided radiation therapy (IGRT) aims at in-room imaging guiding the radiation delivery based on instant knowledge of the target location and changes in tumour volume during treatment. Advancements are usually not to be attributed to a single event, but rather a combination of many small improvements that together enable a superior result. Image-guidance is an important link in the treatment chain and as such a major factor in this synergetic process. A historic review shows that many of the so-called new developments are not so new at all, but did not make it into mainstream radiotherapy practice at that time. Recent developments in improved IT infrastructures, novel irradiation techniques, and better knowledge of functional and morphologic information may have created the need and optimal environment to revive the interest in IGRT.