The complement system is a complex system involving serum and membrane proteins interacting in a regulated manner. The complement system plays a major role in antibacterial immunity, in inflammation, and in immune complex processing. Therefore, deficiencies in complement proteins are associated with increased susceptibility to bacterial infections and autoimmune diseases. These deficiencies can be inherited or acquired. Most of them can be screened by simple laboratory tests but require a diagnosis in a specialized laboratory. All sequences of complement genes are known, and the discovery of a deficiency must lead to genetic testing. The discovery of a congenital deficiency requires a familial study and a prophylaxis. In this article, we review the complement cascade, the laboratory tests to explore it, and the main diseases associated with complement deficiencies.