Nature is the main source of anticancer agents with about 60% of the current anticancer drugs originating in some way from natural products. Many cytotoxic natural products have been isolated from marine invertebrates. One group of marine animals that have made significant contributions is the tunicates or ascidians. Ascidians belonging to the family Didemnidae are known to be a prolific and rich source of new chemical entities with biological activity. This study was divided into two main components. In the first part, Didemnid ascidians collected from the Great Barrier Reef (GBR) were investigated for their chemical diversity using spectroscopic and spectrometric techniques. In part two, an ascidian drug discovery screening library was generated and subsequently used to identify cytotoxic or cytostatic compounds in prostate (LNCaP) and breast (MDA-MB-231) cancer cells. The ascidian natural products isolated in part 1 were all tested in these cancer cell lines. Moreover, one of the cytotoxic compounds identified from the screening studies was subjected to mechanism of action studies.