A display can appear cluttered because it shows a lot of information, the information is irrelevant for the task at hand or the information is poorly organized. In this paper, we review past research on pilot's performance under various levels of visual clutter. We found that this research lacks a common rationale to manipulate clutter and we suggest such a rationale. We show that models defining clutter as the number of elements shown on the display fail to explain recent empirical results where displays with minimal clutter do not give the best technical flight performance. The paper ends with three suggestions to de-clutter a display.