Complex communication needs exist when individuals cannot meet their daily communication needs through their current methods of communication. Given the fact that people with complex communication needs cannot communicate in conventional ways of communication, the development of augmentative and alternative communication system (AAC) has enabled them to communicate in a different way. Augmentative and alternative communication system is an integrated group of components – symbols, aids, strategies, and techniques - used by individuals to enhance communication. Studies have shown that people with complex communication needs have difficulties in all language components. Consequently, improving language skills through a system of augmentative and alternnative communication in people with complex communication needs is crucial. Language skills are essential for literacy development. For people who rely on AAC, literacy skills facilitate successful participation at multiple levels across a variety of environments and provide access to a range of sophisticated AAC systems that allow them to express their thoughts through a combination of symbols and orthography. Literacy acquisition is also difficult for people with complex communication needs. Therefore, in people with complex communication needs, it is important to improve literacy in addition to the language. In the context of AAC, it is important to distinguish core vocabulary and fringe vocabulary. Core vocabulary consists of words and messages that often appear in a language. The words of the core vocabulary make 80% of the words used in everyday communication and are essential for language development. Fringe vocabulary consists of words that are less likely to occur and use than the words of the core vocabulary. These are words that reflect an individual’s activities, interests, environment, personal style, and age group membership. Since the core vocabulary is essential for language development, the AAC systems should contain mostly core vocabulary, although the fringe vocabulary should not be ignored as it is a reflection of user's personality.