Transplant-associated thrombotic microangiopathy (TA-TMA) is a relatively common complication of hematopoietic stem cell transplantation. Th e pathophysiology is complex, the central feature is endothelial injury. Th e role of complement activation in the pathophysiology has been increasingly recognized. Th e diagnostic procedures and interpretations vary considerably between centers, and aims at standardizing the criteria of this complication are important especially for study purposes. Th e therapy is improving, particularly due to the introduction of complement inhibitors. However, the indications and policy of administration of these drugs are not well established and need further study.