Hemolytic uremic syndrome (HUS) is the clinical triad of thrombocytopenia, anemia, and acute kidney injury. Classically associated with enterocolitis from Shiga toxin–producing Escherichia coli, HUS is also associated with Streptococcus pneumoniae infections; genetic dysregulation of the alternative complement pathway or coagulation cascade; and, rarely, a hereditary disorder of cobalamin C metabolism. These share a common final pathway of a prothrombotic and proinflammatory state on the endothelial cell surface, with fibrin and platelet deposition. Much work has been done to distinguish between the different mechanisms of disease, thereby informing the optimal therapeutic interventions for each entity.