Marine organisms are rich sources of structurally diverse bioactive compounds with various biological activities, and their importance as a source of novel bioactive substances is growing rapidly. Marine natural products offer an abundant source of pharmacologically active agents with great chemical diversity and complexity and the potential to produce valuable therapeutic entities. The realization of this potential through the recent approval of two marine natural products has taken many decades (Glaser and Mayer 2009). With marine species comprising approximately half the total global biodiversity, the sea offers an enormous resource for novel pharmaceuticals. Recently, their value as a source of novel bioactive substances has grown rapidly and researchers have revealed that marine-originated compounds exhibit various biological activities. Although marine organisms are exposed to adverse environmental conditions such as light and high-oxygen concentrations that lead to the formation of free radicals, and other strong oxidizing agents, they do not undergo any serious photodynamic damage. Thus, it is known that marine organisms are able to generate bioactive compounds to protect themselves from external factors such as ultraviolet (UV) radiation, stress, and herbivores. Recently, scientists have explored various health-benecial pharmaceutical bioactives from marine bioresources such as macroalgae, microalgae, fungi, bacteria, actinomycetes, invertebrates, and vertebrates. Among marine organisms, marine algae are rich sources of bioactive compounds with various biological activities. Moreover, marine-derived fungi have become an important source of novel pharmacologically active secondary metabolites and are considered as a rich source of bioactive compounds with medicinal and pharmaceutical benets. This chapter discusses the recent trends and ndings associated with novel pharmaceutical bioactives from marine organisms.