Developmental science, cognitive psychology, and philosophy have long struggled with questions regarding ontogenetic relations between thought and language. A key theorist in this regard, whose work has become increasingly influential in the decades since it was initially translated from its original Russian, is L. S. Vygotsky (e.g., 1934/1987). Vygotsky proposed that a fundamental transformation of the child's cognitive processes begins toward the end of the second year of life, when preintellectual language and prelinguistic cognition fuse to create verbally mediated thought. Central to Vygotsky's theory was the claim that inner speech (or verbal thought) originates from linguistic exchanges with others and passes through an intermediate stage of self-directed speech before becoming fully internalized. This self-directed, frequently self-regulatory speech has become known as private speech (Díaz & Berk, 1992; Flavell, 1966). It is held to underpin children's developing mastery over their own behavior, and to provide a window onto the internalization process that, for Vygotsky, is crucial to the formation of higher forms of cognition.

The past decade or so has seen renewed interest in Vygotsky's ideas about private speech, partly as a result of a general increase in enthusiasm for his theory (e.g., Berk & Winsler, 1995; Lloyd & Fernyhough, 1999a–d; Montero, 2006; Winsler, 2003). However, interest in the role of language in executive functioning and in regulating children's behavior is not limited to the Soviet tradition. Currently four relatively independent research literatures, originating from different theoretical traditions, have been making advances in our understanding of the role of language in children's behavioral and cognitive self-regulation.