Primary immunodeficiency diseases (PIDs) are genetically determined disorders of the immune system resulting in greatly enhanced susceptibility to infectious disease, autoimmunity and malignancy. While individual PIDs are rare, as a group, it is estimated that between 1:2000 and 1:10 000 live births are affected by a PID. Moreover, PIDs can present at any age from birth to adulthood, posing a considerable challenge for the practising physician to know when and how to work-up a patient for a possible immune defect. In this review, we outline the basic organisation of the human immune system and the types of infections that occur when elements of the immune system are dysfunctional. Importantly, we provide practical guidelines for identifying patients who should be referred for assessment of possible immunodeficiency and an overview of screening investigations and effective therapeutic options available for these patients.