Activation of the complement system (C) is an important aspect of the immune reaction, and therefore also of inflammatory response. This review summarizes the known pathways for C activation, including the relatively recently described lectin pathway. We must remember that the complement, like the immune response itself, is a two-edge sword. One side is beneficial to the host, aiming to eliminate pathogens, either free or attached to cells. On the other side, overactivation of C may lead to the destruction of tissues, which is obviously harmful. Thus activation of C must be regulated at each stage of the cascade (early, intermittent, and final), and to this end, act specific inhibitors, which often exist as cellular receptors. A list of diseases in which the complement is over-activated was presented, together with a summary of the currently tested or in-use cascade inhibitors. The most dangerous conditions include ischaemias--heart infarct and brain ischaemia. The crucial point in inhibiting C is the possibility of reducing the area of infarct or ischaemia by fast and effective intervention, aiming to disrupt the cascade chain. It is known that excessive killing by C activation is amplified in ischemic tissue; but that the inhibitors may offer protection. One should be aware that many of these exciting studies are being carried out on animals. However, existing knowledge on the efficacy of C inhibitors should be considered jointly with the current use of statins in human therapy, which, among their pleiotropic actions, also lower the level of C activation.