Three fourths of our planet is covered with water, which is not only the origin of life but also a rich source of living creatures. It turns out that around 15% of biological species (about 280,000) live in the oceans, which includes more than 2,000,000 species of lower grade organisms (sponges, corals, molluscs, etc.) except for the ordinary creatures such as ﬁsh, shrimp, and seashell, etc. These marine organisms play a crucial role in the marine ecological system, even though they are not familiar to people. Moreover, this distinctive marine ecological system (high pressure, high salt, oxygen and light deﬁciency) resulted in a ﬁerce struggle for existence among diﬀerent kinds of organisms. In order to survive under such a rigorous ecological environment, many marine organisms have to produce a series of small chemical molecules with unusual structures and signiﬁcant bioactivities, namely secondary metabolites. Theses chemical substances are produced to defend the animals and their larvae against predators, and against the settlement of microorganisms like fungi or bacteria, and act as pheromones as well. Recent research revealed that the diﬀerences in living environments of marine organisms and terrestrial creatures led to a tremendous diﬀerence in their biosynthetic approaches for secondary metabolites. Secondary metabolites from marine sources display greater chemical diversity compared with those from terrestrial organisms. Many marine origin compounds have proven by modern pharmacology to be eﬀective therapies against various diseases, which draws great interest from chemists, biologists, and pharmacologists.