Opisthobranchs are marine gastropod molluscs, the shell of which is either reduced or completely absent in the adults. Being loss of physical protection, the survival of opisthobranchs mainly relies on a series of chemical strategies. Most of the animals protect themselves from predators by selecting appropriate diet, and consequently transforming and bio-accumulating some useful dietary metabolites into selected body parts as defensive substances. A few of animals are capable of biosynthesizing the necessary substances to establish their chemical defensive systems. Obviously, chemical studies on constituents of molluscs and their preys can reveal the dietary relationship between them, and suggest the ecology role of the metabolite as a consequence. Due to the related uniform dietary for the same kind of mollusc, chemical investigation on the animals would lend a hand to their taxonomy. Furthermore, the evolved chemical defensive strategy provides us a new approach in searching for the potential bioactive molecules from natural sources. These lines of evidences promoted many ecologists, chemists and pharmacologists to conduct a series of studies on opisthobranch molluscs, which resulted in numerous publications in the past two decades. In this paper, we summarize the studies covering three large groups of opisthobranch mollusks in the latest five years, aiming at giving the readers a brief view of the compounds, as well as their biological activities, isolated from these mollusks, and when it is possible, to suggest ecological roles of them.