There is an immense diversity of marine plants and animals from which an estimated 14,000 pharmacologically active compounds have been isolated. However, in terms of clinically useful anti-cancer agents, the oceans remain as a largely untapped resource. Indeed, there are currently only two compounds used in the clinic that are derived from marine sources. These are cytarabine, which is a deoxycitidine analogue and aplidine, which has both growth inhibitory and anti-angiogenic effects. This situation is likely to change rather dramatically in the near future, as attention has focused on the vast diversity of available agents from marine organisms. The increased pace of activity in this area has resulted in a several clinical trials of promising compounds with the probability that these will be followed by other drugs currently under preclinical development.