The peptidyl prolyl isomerase (PPIase) Pin1 has been recently implicated in cell cycle control and neuropathologies. There is now growing evidence that Pin1 plays an important role in the immune system and does so differentially from the related PPIases, cyclophilinA and FKBP. This review describes how Pin1 modulates cytokine expression by activated T cells and eosinophils and participates in T-cell and eosinophil apoptotic decisions both in vitro and in vivo. We highlight several possible immunologic diseases, including asthma, as well as organ transplant rejection, where anti-Pin1 therapeutics maybe of value.