This entry reviews the definitions, theories, measures, development, characteristics, neuroanatomy, and functions of inner speech, as well as the maladaptive impact of negative self-talk. Inner speech is involved in self-regulation, working memory language functions, and self-awareness. Most assessment tools are introspective (e.g., thought sampling) or retrospective (e.g., questionnaires). Children’s private speech constitutes the best available objective manifestation of inner speech. Inner speech is social in origin, it is preceded by private speech, and once internalized, it becomes abbreviated and predicative. Inner speech production activates the left inferior frontal gyrus. Distorted self-talk is associated with psychological disorders like anxiety and depression.