© 2018 Elsevier Inc. All rights reserved. This chapter discusses the purpose, process, and outcomes of pretransfusion testing in the transfusion service and beyond. During such testing, complications may arise ranging from ABO discrepancies to allo- or autoantibody detection. These complications must then be dealt with by the transfusion medicine team so that the patient can receive the safest blood possible. This chapter aims to explain this process, while also describing the clinical consequences and solutions.