Inner speech is the ability to speak in one´s mind and is implicated in many cognitive functions, including planning, problem-solving and self-regulation in children and adults. Vygotsky´s (1934/1987) theory of inner speech found that the more inner speech is used by children the more efficient executive functions become. This important development step is strongly related to individual speech competencies and cognition. Which process clearly mediates this interplay is still not completely understood. A better understanding of inner speech thus could improve the understanding of both typical and atypical speech and cognitive development. The study Language processing supporting cognitive performance: an investigation of children´s and adults´ inner speech using behavioral, electrophysiological and optical methods will provide important insights into the interplay between language competencies, inner speech, and executive functions in children and adults. We aim at investigating how individual language competencies in 5-6-year-old children are related to executive functions and mediated through inner speech processing, mainly in problem-solving situations. However, the knowledge about the neuronal underpinnings this interplay is scare. In cooperation with the Medical University of Innsbruck (Lab for Cognitive Neuroscience, Department for Hearing, Speech and Voice Disorders) we put under investigations the neuronal correlates of inner speech compared to overt speech. In particular the differentiation between planning and actually executing overt and inner speech production is explored in children of different ages (6-7- and 8-9-year-olds) as well as in adults. We want to disentangle fast occurring mechanisms and identify involved brain areas by simultaneously applying the electroencephalography (EEG) and the functional near-infrared spectroscopy (fNIRS) in adults and children.