Interleukin-17 (now known as IL-17A), is a homodimer of two 155 amino acid chains secreted by CD4+ activated memory T cells (CD45+ RO+) and is available as a glycosylated 20- to 30-kDa homodimeric peptide. Human IL-17 shows amino acid sequence identity of 62.5 and 58% to the mouse and rat sequences, respectively. IL-17 can regulate the function of a variety of cell types, plays an important role in the maturation of hematopoietic progenitor cells, and induces production of proinflammatory mediators. Here, for the first time, we summarize the biological effects of IL-17 and its family members as important players of T cell-mediated immune responses and underline the important implications of this cytokine in inflammation and degenerative diseases.