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This thesis describes the NMR-guided isolation and structural elucidation of one novel and several known compounds from marine organisms collected from Tonga and New Zealand. In the process of this work, 11 Tongan algal specimens were subjected to preliminary NMR-guided investigation, as the study of Tongan marine algae is poorly represented. The HMBC spectra of crude fractions generated by the first chromatographic purification of the crude extracts were analysed for perceived structural novelty, providing three specimens that warranted further investigation. Investigation of unknown algae PTN4_17G afforded substructure 76, observed in the known compound avrainvilloside, which included the rare 6-deoxy-6-aminoglucose moiety. This aminoglucose moiety has been reported from marine sources only twice in literature, both in algae specimens. An investigation of unknown brown algae PTN4_18E afforded substructure 83, a methacrylic acid containing moiety. Methacrylic acid moieties are also uncommon in marine organisms, with 27 reported structures containing the moiety, of which only two occur from algae. In each case suitable, mass spectrometry data was not obtainable therefore full structural elucidation of the compounds was not achieved. Further analysis of the unknown algae PTN3_38C afforded the known compound fistularin-3 86, although further studies revealed that a sponge contaminant was responsible for the presence of the compound. The results of this algae study provided interesting correlations between secondary metabolite concentrations of algae in temperate and sub-tropical environments, contrary to the observed correlations of marine sponges. An investigation into an unknown New Zealand Raspailia sponge was conducted as previous studies had suggested the presence of novel resonances. Further analysis of the specimen yielded the known clerodane raspailodane A 126 and the unexpected novel steroidal glycoside raspailoside A 135. Biological activity studies conducte