Meanings given to the term "perception" are discussed in this paper and two categories of perception theories having implications for instructional media research are identified. One category is described as extroverted because it includes theories primarily concerned with characteristics of external stimuli and the influence that the corresponding information has on the formation of mental representations or percepts. Described as introverted, the second category of perception theories is concerned with the self-directed internal processes that individuals use in constructing percepts from sensory information. Research based on both theories of perception is reviewed as well as the literatures on (1) the ability to mentally construct or transform memorial representations and on (2) the validation of perceptual processes. Several questions for further research are outlined. A figure showing the use of anorthscopic perception (i.e., the ability to recognize figures as they pass behind a narrow slit which is oriented vertically in the center of a stationary opaque field) to validate central mechanisms of perception is included and 64 references are listed. (MES)