Expertise in object recognition, as in bird watching or X-ray specialization, is based on extensive perceptual experience and in-depth semantic knowledge. Although it has been shown that rich perceptual experience shapes elementary perception and higher level discrimination and identification, little is known about the influence of in-depth semantic knowledge on object perception and identification. By means of recording event-related brain potentials (ERPs), we show that the amount of knowledge acquired about initially unfamiliar objects modulates visual ERP components already 120 msec after object presentation, and causes gradual variations of activity in similar brain systems within a later timeframe commonly associated with meaning access. When perceptual analysis is made more difficult by blurring object pictures, knowledge has an even stronger effect on perceptual analysis and facilitates recognition. These findings demonstrate that in-depth knowledge not only affects involuntary semantic memory access, but also shapes perception by penetrating early visual processes traditionally held to be immune to such influences.