Thrombotic microangiopathy (TMA) is a rare but devastating disorder; it involves small vessels and is characterized by intravascular thrombi of aggregated platelets leading to thrombocytopenia and variable degrees of organ ischemia and anemia, which is due to erythrocyte fragmentation in microcirculation. Childhood cases with predominant renal involvement are referred as the hemolytic uremic syndrome (HUS), and adults with major central neurological involvement are labeled as thrombotic thrombocytopenia purpura (TTP). Endothelial damage due to toxins and/or lack of defense against complement activation have a central role. Recent discovery of the von Willebrand Factor cleaving protease (ADAMTS 13) has offered new insight into the pathogenesis of TMA. TMA is also a well-recognized serious complication of renal transplantation. Clinical features of intravascular hemolysis are not always found. It may occur as de novo or recurrent and the majority of de novo cases are related to cyclosporin therapy. Viral infections, severe renal ischemia and acute vascular rejection are less frequent causes. Recurrence is negligible in diarrhea-associated HUS in childhood, but non-diarrheal HUS recurs in majority of adults following renal transplantation. Renal transplantation is contraindicated in familial/relapsing recurrent forms of HUS.