Psoriatic arthritis and spondyloarthritis (SpA) are complex immune-mediated diseases affecting peripheral and axial joints. T cells have been considered fundamental in triggering the disease and maintaining the process in the chronic phase. The recent discovery of the CD4+ Th17 lymphocyte subset and the interleukin 23/interleukin 17 axis has further contributed to the definition of unknown pathways, challenging previous models and the role of Th1/Th2 T cells in immune mediated diseases, including SpA.