Publisher Summary This chapter discusses the adverse effects of the blood and blood products. Due to the great care and attention devoted to blood group determinations and cross-matching, hemolytic transfusion reactions rarely occur. Not every hemolytic transfusion reaction can be prevented. Some patients are sensitized by blood transfusion or pregnancy, but the antibody is weak and escapes notice during cross-matching. If incompatible blood is transfused, an anamnestic response occurs and antibody production may increase rapidly. Diagnostic difficulties can occur if the antibody involved is directed against a high frequency blood group antigen. Hemolytic transfusion reactions are often caused by errors, lack of care and inaccuracy. The chapter provides an account for the screening for (1) red blood cell antibodies, (2) rhesus (D) antigen and antibody, (3) low frequency antigens, and (4) high frequency antigens.