Immunodeficiency refers to a condition in which the immune system's ability to fight infections is weakened or missing. The majority of cases occur because of extrinsic factors, which impact the patient's immune system, such as nutrition and HIV infection. Primary immunodeficiency (PID) is a disease in which a portion of the body's immune system is absent, or is not able to function properly. A condition will be considered PID if the cause of immune deficiency is not secondary in nature, such as a condition caused due to drug treatment or other disease. The diagnosis of PID includes evaluation of immunoglobulin levels and a full blood cell count. The treatment of these diseases relies on the severity of the abnormality. Majority of the treatments are palliative and passive, which can be categorized into two modalities, which include boosting the immune system and managing infections. This book contains a detailed explanation of the various diagnostic and management techniques associated with primary immunodeficiency diseases. Those in search of information to further their knowledge will be greatly assisted by it.