Current transfusion therapy guidelines rely, in part, on hematology tests such as hemoglobin, hematocrit, prothrombin time (PT), and activated partial thromboplastin time (aPTT). However, what do all of these tests actually measure, and what do the results mean? After a basic review of the different tests done in hematology, the indications for blood products will be presented. Cases will be used to illustrate the concepts.