For the optimization of digital imaging systems it is crucial to known how parameter settings affect the perceptual quality of displayed images. This calls for valid techniques for assessing image quality. Here, we studied continuous assessment of the instantaneous quality impression of long image sequences. Initially, we concentrated on the measuring method. Subjects were instructed to indicate quality by mooing a slider along a graphical scale. With a sequence consisting of time-variably blurred stills the temporal characteristics of continuous scaling could be separated from the relation between blur and quality impression. The temporal behavior can be explained by a causal linear time-filter. Subsequently, we extended the method to real video. In order to check the validity of continuous scaling, perceived quality of the video at any moment in time was measured by partitioning the video in short fragments and evaluating the quality of each fragment separately. The image material was MPEG-2 coded at 2 Mbit/s. The relation between the time-quality curves from the continuous assessment and the instantaneous ratings of the fragments is described by the same time-filter as found previously. This filter indicates a delay of 1 second, and suggests that subjects can monitor image quality variations almost instantaneously. With these experiments, we have shown that it is possible to measure quality of video sequences continuously in a consistent way. As confirmed in a third experiment, the results of continuous assessment give the possibility to select relevant material for further analysis, for instance by standard ITU/R methods.