This document presents a theory of how the psychology and neurobiology of stimulus appraisal influence variability in second language acquisition, and extends the notion of affect developed for second language acquisition to primary language acquisition and to cognition in general. The first chapter lays out a psychological framework that develops the notions of value, emotional memory, and stimulus appraisal. Chapter 2 suggests a neurobiological mechanism that would subserve the psychological constructs just described. Chapters 3 through 5 offer evidence for the idea that stimulus appraisal constitutes the affective basis for motivation in second language learning and all deep, sustained learning. Affect in first language acquisition and use is explored in chapter 6, arguing that the affective relationship between caregiver and child forms the basis for language acquisition. The final chapter explores the role of affect in cognition in general. Contents are indexed. (MSE)