This thesis is divided into two parts (A and B). Our overall aim was to isolate and characterize natural products from Australian marine sponges. The uniqueness of this thesis arises from the application of Density Functional Theory (DFT), a quantum mechanical modelling method, which can be used to help elucidate and confirm molecular structures. DFT calculations, and in particular, DFT-NMR (Nuclear Magnetic Resonance) calculations, are still in their relative infancy. The research presented in Part A exemplifies DFT-NMR as a powerful technique, which can be added to a natural product chemist’s “structure-elucidation toolkit”.