Nudibranchs are a group of colourful, shell-less marine molluscs that have evolved alternate methods of defence against predators, such as stealing stinging nematocysts cells from their coral prey or sequestering deterrent toxins from the marine sponges they feed upon. Many chemically defended nudibranchs also display bright colour patterns to warn predators of their unprofitability, a phenomenon known as aposematism. The chemical defences of nudibranchs offer a rich source of bioactive molecules with high pharmaceutical potential, but the evolutionary origin and adaptations encompassing the defensive chemical ecology of nudibranchs have received less attention. This thesis explores the broad chemical diversity of various aposematic nudibranchs to better understand the underlying mechanisms and modes of anti-predator defences. In Chapter 2, I first established whether brine shrimp toxicity assays can viably assess the defence levels of nudibranch metabolites. Brine shrimp are often used as model organisms for preliminary toxicity screening, but whether the results from these assays are ecologically meaningful in comparison with more relevant predators (e.g. visually hunting predators in the case of aposematic nudibranchs) remain uncertain. I used various nudibranch groups that utilise different chemical types to obtain a range of toxicity levels and compared the relative toxicity of these compounds against brine shrimp and a potential fish predator (the blue-green damselfish, Chromis viridis). I found that nudibranch extracts toxic to brine shrimp were also toxic to fish, but extracts that were innocuous to brine shrimp may still be toxic to fish. The results from this chapter confirmed that brine shrimp toxicity assays can reasonably indicate nudibranch toxicity levels, but further assays with fish are required when no response is observed against brine shrimp. In Chapter 3, I explored how nudibranchs employ rich chemical mixtures as a defence strategy against a variety of predators. I studied 14 nudibranch species from nine different genera and four families, which yielded a range of defensive mixtures such as spongian furanoditerpenes from Doriprismatica atromarginata, spongian and rearranged diterpenes from Goniobranchus spp. and dior sesquiterpenes from Phyllidiid nudibranchs. I determined the strength and modes (toxicity and unpalatability) of the different chemical types via toxicity assays with brine shrimp, incorporating the results from the previous chapter, and unpalatability assays with two generalist marine shrimp and fish predators. Generally, most nudibranch extracts were highly unpalatable to both fish and shrimp but toxicity to brine shrimp varied. With these results, I discuss the effectiveness of each modality in deterring predators and the various anti-predator strategies employed by different nudibranch species. In Chapter 4, I ascertained the chemical profiles of various Chromodoris species. Firstly, I established whether other Chromodoris species also utilise the same defensive strategy of selectively sequestering latrunculin A in the rim tissue as has been shown in C. kuiteri, C. elisabethina, C. magnifica and C. lochi. I found that C. westraliensis and C. willani may also sequester this metabolite, but the sample size was small (n=4 and 2, respectively). C. colemani and C. burni extracts contained latrunculin A on rare occasions, but the complex chemical profiles of these species indicate that the metabolite is not selectively accumulated. Other species (C. striatella, C. cf. striatella QLD, C. cf. burni, C. strigata) did not utilise latrunculin A and were instead defended with a range of oxygenated sesquiterpenes, spongian and rearranged diterpenes or scalarane and lactone sesterterpenes. For the species that selectively sequestered latrunculin A, I explored the intra- and interspecific variation in the metabolite amounts harboured. C. kuiteri had the largest median concentration, while similar levels were observed between C. lochi, C. magnifica and C. elisabethina. C. kuiteri also displayed the largest within-species variation in metabolite levels, which may correlate negatively to body size according to preliminary results. Finally, in Chapter 5, I explored the storage of latrunculin A within specialised reservoirs, Mantle Dermal Formations (MDFs), as a potential method to prevent auto-toxicity given the high toxicity of this metabolite. I used C. elisabethina, C. kuiteri and/or C. magnifica to investigate various aspects of this storage mechanism, including: (i) the anatomical distribution of latrunculin A; (ii) the potential correlation between metabolite levels and number of MDFs within an individual; and (iii) the visualisation of the metabolite within the MDFs via mass spectrometry imaging techniques. Taken together, the results from this chapter demonstrate that (i) the concentration of latrunculin A is highest in the rim and is dependent on the amount in the viscera, so it is likely that the metabolite is stored directly in the MDFs upon consumption; (ii) the lack of correlation indicate that MDFs have likely evolved for defensive purposes i.e. the delivery of a concentrated dose to deter more persistent predators, rather than to avoid auto-toxicity; and (iii) latrunculin A is localised precisely within the MDFs, and mass spectrometry imaging can be a powerful tool for further work.