Abstract
It is probable that nearly every natural product structure results from interactions between organisms. Symbiosis, a subset of inter-organism interactions involving closely associated partners, has recently provided new and interesting experimental systems for the study of these interactions. This review discusses new observations about natural product function and structural evolution that emerge from the study of symbiotic systems. In particular, these advances directly address long-standing 'how' and 'why' questions about natural products, providing fundamental insights about the evolution, origin and purpose of natural products that are inaccessible by other methods.