Various active anticancer agents are derived from plants and terrestrial microorganisms. The isolation of C-nucleosides from the Caribbean sponge, Cryptotheca crypta, four decades ago, provided the basis for the synthesis of cytarabine, the first marine-derived anticancer agent to be developed for clinical use. Cytarabine is currently used in the routine treatment of patients with leukaemia and lymphoma. Gemcitabine, one of its fluorinated derivatives, has also been approved for use in patients with pancreatic, breast, bladder, and non-small-cell lung cancer. Over the past decade, several new experimental anticancer agents derived from marine sources have entered preclinical and clinical trials. This field has expanded significantly as a result of improvements in the technology of deep-sea collection, extraction, and large-scale production through aquaculture and synthesis. In this paper, examples of marine-derived experimental agents that are currently undergoing preclinical and early clinical evaluation are briefly discussed. A summary of the available information on the results of phase I and II trials of agents such as aplidine, ecteinascidin-734 (ET-734), dolastatin 10 and bryostatin 1 is also presented.