Complement is vital for protecting individuals against pathogens and any disturbance of homeostasis associated with appearance of foreign antigens. Four antenna molecules seek for putative danger and subsequently start three activation pathways to eliminate the hostile triggering signal. To achieve this mission the complement arsenal contains soluble plasma factors as well as membrane-bound receptor molecules. Fulfilling a broad spectrum of biological functions, complement participates to construct and orchestrate an immunological network with extensive links to other elements of innate immunity, but also to its younger brother, the adaptive immune system. The body generously supports the complement activity with a high level of complement production; not only the liver as 'the capital of complement expression' but also decentralized synthesis sites guarantee its all-over presence. On the other hand, it is of fundamental interest for the organism to limit this powerful immunological regiment by establishing a tight surveillance composed of redundantly acting regulator molecules. To find the appropriate dimension of complement activity is critical, as shown by the spectrum of diseases associated with an excess or a lack. Numerous therapeutic approaches aim to correct such an imbalance and to re-establish the antimicrobial capacity of complement without induction of chronic inflammation and autoimmunity.