It is often suggested that people visualize information in order to improve their comprehension, but in some cases this can cause problems in viewing. I will review several experiments which show that visual threshold for small targets can be adversely affected by (mental) visual imagery. This research goes back to E. W. Scripture (Science, 1896) and C. W. Perky (1910), but was largely forgotten. Our work shows that mental visual images 'prjected' in the same depth plane or in front of a visual target lower visual acuity; when projected behind, they do not. Only close-in images have an effect; distant ones have no effect, and nor do auditory images, showing that the loss of acuity is not merely due to the effort involved in maintaining a mental image. Controls show that the effect is not due to poor accomodation of the eye or to poor eye movements; nor is it due to the distraction of attention; it appears to be a purely perceutal effect arising in the visual pathway.