Jones A, Heyes J (2014) Blood transfusion 2: processing, testing and selecting blood components. Nursing Times; 110:37,20-22. Transfusion of blood components can be an essential and lifesaving treatment for many patients. However, components must comply with a number of national requirements to ensure they are safe and fit for use. Transfusion of incorrect blood components can lead to mortality and morbidity in patients, which is why patient testing and blood selection are important. This second article in our five-part series on blood transfusion outlines the requirements for different blood components, the importance of the ABO and RhD blood group systems and the processes that ensure the correct blood component is issued to each patient.