The present invention provides derivatives of squaric acid, in particular derivatives of 3,4-diamino-cyclobut-3-ene-1,2-dione and tautomers and isomers thereof, as a single stereoisomer or a mixture of stereoisomers, or as a pharmaceutically acceptable salt thereof. These compounds show anti-proliferative activity, including against tumor cells, and are useful in the treatment of diseases including cancer.