Current research reveals that individuals use two languages: external language and internal language. The external language is a loud, noisy language that everyone can hear. The internal language is a silent, mysterious, personal language that merely the individual hears. This phenomenon, called inner language, inner voice or inner speech, is a mental activity seen in most people. This activity plays important roles in language and cognitive processes, helps the processes such as reading, writing, understanding, questioning, learning, thinking, controlling emotions, motivation, and so on. The inner voice emerges during silent reading and contributes significantly to understanding words, sentences, and text. Expert readers read and understand the words by hearing the small inner sound of the words in their brain. Controversy, internal speech is when an individual talks silently to himself. Inner speech is referred when planning daily tasks, making decisions, solving problems, and leading social and emotional issues. Nonetheless, research on the inner voice and inner speech, which are effective in the development of the individual, is yet insufficient. There is a need for clearer and more comprehensive research on the role and function of inner speech in processes such as reading, speaking, understanding, and thinking. It is recommended that this phenomenon, which exists in every individual, be handled scientifically and used to develop language and cognitive skills.