Primary immunodeficiency diseases (PID) are a group of disorders involving defects in one or more components of the immune system, and are characterized by an increased incidence of infections, autoimmunity, and malignancies. Although PID seem to be rare, the number of patients diagnosed has increased in recent years, and more than 150 different forms of PID have been identified. Nevertheless, because of inadequate medical awareness, a significant number of patients with PID are either not recognized as having a PID or are not diagnosed as early as they should be. Such delays lead to a substantial increase in morbidity and mortality among affected individuals. Our understanding of PID is improving rapidly, which will hopefully lead to more accurate diagnosis and efficient disease management. This book contains the most recent advances in the field, as well as a concise and structured review of previously identified PID. Although the book primary focus is on practical diagnosis and management, the pathophysiology of PID is also discussed. This book is a comprehensive yet manageable resource for physicians and nurses wishing to learn more about PID, as well as a useful tool for both doctors-in-training and specialists in clinical decision-making and treatment planning.