This article reviews the literature on sensory processing sensitivity (SPS) in light of growing evidence from evolutionary biology that many personality differences in nonhuman species involve being more or less responsive, reactive, flexible, or sensitive to the environment. After briefly defining SPS, it first discusses how biologists studying animal personality have conceptualized this general environmental sensitivity. Second, it reviews relevant previous human personality/temperament work, focusing on crossover interactions (where a trait generates positive or negative outcomes depending on the environment), and traits relevant to specific hypothesized aspects of SPS: inhibition of behavior, sensitivity to stimuli, depth of processing, and emotional/physiological reactivity. Third, it reviews support for the overall SPS model, focusing on development of the Highly Sensitive Person (HSP) Scale as a measure of SPS then on neuroimaging and genetic studies using the scale, all of which bears on the extent to which SPS in humans corresponds to biological responsivity.