The chemical and biological diversity of the marine environment is immeasurable and therefore is an extraordinary resource for the discovery of new anticancer drugs. Recent technological and methodologic advances in structure elucidation, organic synthesis, and biological assay have resulted in the isolation and clinical evaluation of various novel anticancer agents. These compounds range in structural class from simple linear peptides, such as dolastatin 10, to complex macrocyclic polyethers, such as halichondrin B; equally as diverse are the molecular modes of action by which these molecules impart their biological activity. This review highlights several marine natural products and their synthetic derivatives that are currently undergoing clinical evaluation as anticancer drugs.