Ruxolitinib, formerly known as INCB018424 or INC424, is a potent and selective oral inhibitor of JAK1 and JAK2. Ruxolitinib has been approved for the treatment of myelofibrosis, which is characterized, biologically, by the activation of the JAK-STAT pathway and, clinically, by bone marrow fibrosis, splenomegaly, abnormal blood counts, and poor quality-of-life through associated symptoms. Ruxolitinib treatment results in a meaningful reduction in spleen size and symptom burden in the majority of myelofibrosis patients, and it may also have a favorable effect on survival. Treatment response apparently does not depend on the presence of a JAK2 V617F mutation. The predominant toxicities are thrombocytopenia and anemia. The metabolization of ruxolitinib through CYP3A4 needs to be considered particularly if co-administered with potent CYP3A4 inhibitors. Several further JAK inhibitors are currently studied in myelofibrosis or other immuno-inflammatory diseases.