Interleukin 17 (IL-17) is the group of proteins known as pro-inflammatory cytokines produced mainly by activated CD4+ T helper (Th) cells. The family includes six members: IL-17A, B, C, D, E (known as IL-25) and IL-17F. The discovery of this cytokine provided a new light into immunoregulation, host defense as well as the pathogenesis of some immunological disorders. It was also noticed that it may play a role in allograft rejection. It is believed that IL-17 stimulates production of different cytokines like TNF-α, IL-1β, IL-21 and IL-6 or chemokines which was influence on neutrophils and macrophages, their recruitment, activation or migration. Specific transcription factors are needed for their differentiation, e.g. TGF-β, IL-6 or IL-23. The aim of our work was to introduce the current knowledge about IL-17 family and its presence or participation in the development of inflammation (e.g. inflammatory bowel disease, infection of Helicobacter pylori) and immunological diseases such as e.g. allergic asthma or rheumatoid arthritis. This study is based on clinical and experimental observations shown in the literature.