This paper examines the validity of Vygotsky's interpretation of the private speech (PS) phenomenon on the basis of findings reported in the literature. A review of available findings provided support for an age-related shift from overt to covert means of verbal self-regulation and for an interpsychological to intrapsychological shift in the regulation of problem-solving behaviors. Several studies reported increases in self-regulatory PS following increases in task difficulty. Although correlational data suggested that these increases were associated with more successful task performance, further support for a self-guidance function was absent in studies that more directly examined the degree of correspondence between overt verbalizations and problem-solving behaviors. Relatively little support was found for the curvilinear developmental hypothesis. A lack of self-regulatory PS was not a distinctive feature of impulsive children, who instead exhibited more task-irrelevant PS. Inconsistent findings hindered an interpretation of the influence of others upon the occurrence of PS. The relative absence of pertinent data precluded any appraisal of whether PS undergoes developmental changes in structure. While the above findings provided some basis for questioning the overall validity of Vygotsky's theoretical framework, this conclusion was tempered by a consideration of several methodological difficulties noted among the reviewed studies. (Author/RH)