Thrombotic thrombocytopenic purpura (TTP) is a rare, life-threatening disease characterized by acute episodes of widespread microvascular thrombosis. The discovery that the plasmatic activity of the von Willebrand factor cleaving protease, ADAMTS13, is severely deficient in patients with TTP partially clarified the pathophysiology of the disease. However, the finding of severe deficiency of ADAMTS13 alone is unable to fully explain the clinical heterogeneity of the disease. The recent development of methods that measure ADAMTS13 activity with great analytical precision offers the opportunity to define the relationships between levels of ADAMTS13 activity below 10% (herein defined as "residual ADAMTS13 activity") and the clinical manifestations of the disease. Recent studies suggest that the amount of residual activity of ADAMTS13 may be a major determinant of the clinical heterogeneity of TTP. Herein, we review the recent findings on residual ADAMTS13 activity and their implications for research and clinical practice in the field.