Factor XIII (FXIII) is a transglutaminase consisting of 2 catalytic A subunits and 2 noncatalytic B subunits in plasma. The noncatalytic B subunits protect the catalytic A subunits from clearance. Congenital FXIII deficiency may manifest as a lifelong bleeding tendency, abnormal wound healing, and recurrent miscarriage. Acquired FXIII deficiency, with significant reductions in FXIII levels, has been reported in several medical conditions. The routine screening tests for coagulopathies-prothrombin time, activated partial thromboplastin time, and thrombin time-do not show abnormalities in cases of FXIII deficiency. A quantitative, functional, FXIII activity assay that detects all forms of FXIII deficiency should be used as a first-line screening test. Treatment consists of recombinant FXIII or FXIII concentrate. If these are unavailable, then fresh-frozen plasma and cryoprecipitates may be used. Factor XIII has a long half-life; therefore, the patients can lead near-normal lives with regular replacements. Patients with acquired FXIII deficiency with inhibitors need immunosuppressive therapy in addition to factor replacements.