Comparative studies utilizing avian and mammalian embryos and neonates have proven particularly useful in exploring how alterations in sensory experience during the perinatal period can affect subsequent development. This article reviews research drawn from comparative developmental psychobiology and concludes that the effects of modified sensory stimulation on perceptual and behavioral development depend on several related factors, including the timing of stimulation relative to the developmental stage of the young organism, the overall amount of sensory stimulation provided or denied, and the type of sensory stimulation presented. Directions for future research on the care of the high-risk infant are discussed.