The purpose of the thesis was to find out teachers' perception of the phenomenon of alternative and complementary communication (AAC) and how AAC enables pupils to develop their communicative and linguistic abilities. The study has a qualitative approach and it is based on a phenomenographic research approach and a socio-cultural perspective. The empirical data collection was done through interviews with eight teachers. The teachers work at or have experience of special needs schools for pupils with intellectual disability and pupils who lack verbal communication.  To analyze the material, thematic analysis was used. Through the interviews, it emerged from the teachers' statements that the phenomenon of AAC supports pupils who lack verbal communication to learning and social interactions. This, in turn, enables participation and independence. Based on the analysis of the results, four levels of description categories of AAC were made visible. Description category one is the most basic level and description category four the most advanced. Based on the results of the study, it is made visible that students should be offered communicative and linguistic development through the support of individualized AAC, practiced on their communicative and linguistic abilities based on the proximal development zone. It emerged from the result that categories one and two do not develop students' linguistic abilities in a qualified way, since within these categories of description, communication consists primarily of one-word sentences and does not contain grammar, word classes or sentence structures. Under category one you find AAC methods where students are offered communication through single images and objects.  Categories three and four include communication tools that contain linguistic development possibilities, such as alphabets, text, written language and speech synthesis that enable multi-word sentences. Based on the informants' statements, the need for AAC methods that develop the students' linguistic abilities is made visible. It also emerged that the collaboration with habilitation and speech therapists was a crucial factor in being able to offer students communication tools from categories three and four.