Quinones still comprise one of the largest classes of antitumour agents. For example, the anthracycline antibiotics are among the most utilised anticancer agents ever developed. Many other quinones were tested for their anticancer activity. Though there are general and well-established mechanisms for quinone toxicity, the exact contribution of the quinone moiety to the cytotoxic effect remains frequently uncertain. However, DNA represents the main target for quinoid antitumour agents and most of them belong to the groups of DNA intercalating and/or alkylating agents. But also other cellular structures such as heat shock protein 90 or telomerase have been identified as targets for quinoid compounds.