Tight oligopolies are oligopolies the market characteristics of which facilitate the realisation of supranormal profits for a substantial period of time. We entangle the link between market structure and the possibility of welfare reducing behaviour by firms. A useful distinction can be made between â€˜unilateral effects' (oligopolistic firms realise supra-normal profits without co-ordinating their strategies) and â€˜co-ordinated effects' (oligopolistic firms realise supra-normal profits by co-ordinating their strategies). The study develops a â€˜diagnostic approach', a tool that helps policy makers find proportionate remedies to tight oligopolies: (1) â€˜prevent' a market from becoming a tight oligopoliy; (2) â€˜cure' a currently tight oligopoly by changing the market structure; and (3) treat the symptoms of an established tight oligopoly. We apply this diagnostic approach to six cases of (potentially) tight oligopolies.