Blood transfusion is considered a life-saving therapy since ancient times, but, at the same time, a high-risk procedure. Nowadays the common perception is that infection is the greatest risk, even if the blood has never been safer from this point of view. Currently, the residual risk of transfusion must be related mainly to immunological mechanisms underlying to AB0 and minor blood systems, to compatibility of blood transfused and to development of irregular antibodies in transfused patients. "Transfusion Medicine and Patient Safety" aims to provide the basic of immunohematology to readers and to analyze the transfusional process highlighting the most critical points, thus more exposed to errors. Screening on blood and blood components for infectious diseases along with the surveillance action on emerging viruses results in the drastic reduction of post-transfusion infection, together with the potential to further increase the level of security from infection through the inactivation of blood components. The text also describes the major diagnostic systems and organizational models that modern technology provides us with a correct immunohematological diagnosis and an appropriate trasfusional therapy.