It appears the state of the art of evaluating factors VIII and IX is in its infancy. With the exception of the screening tests, the more specific methodologies are expensive, time-consuming, and require a certain amount of technical proficiency for proper performance. However, our ability to evaluate these molecules has advanced greatly since the early 1970s. Most probably the methods described in this paper will be available in the near future to all laboratories wishing to perform them, either in kit form or in a more simplified version. Meanwhile, the APTT, mixing tests, qualitative factor assays, and bleeding times may be performed by nearly any laboratory. The performance of these tests will help identify those patients who may have potentially life-threatening postoperative bleeding episodes or who are having a bleeding episode that requires diagnosis for effective treatment. It is therefore important that these tests be incorporated into our routine coagulation protocols.