Infections constitute the most common and severe complication of immune deficiencies. The pattern of infections is strikingly dependent on the type of immune defect. Antibody deficiencies usually manifest with recurrent capsulated bacteria related-infections of the respiratory tract, due to pneumoccoci and Haemophilus influenza. The same bacteria are implicated in asplenic patients with frequent sepsis of devastating consequences. Complement deficiencies must be looked for in case of meningococcal infections. Neutropenia and defects in phagocytic cell functions favour bacteraemia and invasive fungal infections. Importantly, neutropenia can rapidly lead to septic shock. Cellular immune deficiencies are associated with opportunistic infections including viral infections due to Herpes viridae, fungal and parasitic infections (Pneumocystis jiroveci, Toxoplasma gondii) and mycobacterial infections. Most of the time, immune defects are combined, accounting for the variety and the complexity of clinical presentations and microbial investigations.