Nature is an important source for novel drugs. In fact nearly half of the currently marketed cancer drugs are derived from natural products. Plant-derived cytotoxic compounds such as paclitaxel, vincristine, epothilones, doxorubicin, etoposide, and camptothecin are all widely used in cancer therapy today. More recently, the oceans have received growing interest as a source for new useful bioactive compounds. The marine environment comprises the majority of the global biodiversity. The wide-ranging genetic variation has engendered unique biosynthetic pathways that produce structurally diverse small organic molecules that extend the chemical space. The huge variety of the structures present in marine organisms has been illustrated by the couple of successful molecules used in the clinic like: Cytarabine, Vidarabine, Ziconotide, Eribulin Mesylate and Trabectedin and plenty of others in ongoing clinical trials.