The complement system is part of the innate immunity. It is a multifunctional system including more than 30 plasma and membrane proteins. These are activated by an enzymatic cascade and proteolytic reactions producing activating fragments. Complement is : 1) among the first line of defense towards a pathogen, 2) increasing efficacy of the acquired immunity, 3) responsible for the elimination of immune complexes apoptotic and or necrotic cells (waste removal function) and many other cellular and tissue functions. The aim of this article is to analyze the consequences of complement dysregulation in the development of various diseases.