Crude oils are natural substances derived from the degraded remains of organic matter and are typically complex and highly variable in nature. Toxicological assessment following an oil spill is challenging not only due to the variable chemical characteristics as a slick spreads and weathers, but also to the vulnerability of the affected ecosystem. Rapid decisions must be made to determine how to respond to the spill to protect the environment. Hence, a rapid assessment toolkit specific to the ecotoxicological assessment of crude oil and its components has been recommended. This thesis aimed to develop such a rapid assessment toolkit based on state of the art, environmentally relevant, diagnostic chemical and biological tools. Challenges to be addressed included the choice of test species, the dosing method for ensuring a consistent exposure concentration and the extrapolation of the data to predict species sensitivity and ecosystem relevance using computational modelling tools. To determine the applicability and comparative sensitivity of cyst-based zooplankton survival assays (using brine shrimp Artemia franciscana and marine rotifer Brachionus plicatilis) a novel, miniature passive dosing method was developed. Using three exemplar aromatic hydrocarbons, brine shrimp and rotifer were found to be within 51- 75% and 76–100% of most resistant species respectively when compared to 79 other species. When these species sensitivities were extrapolated using computational modelling (PETROTOX) to predict the relative toxicity of crude oil, they provided a conservative estimate when compared to empirical data derived from laboratory tests. In addition, comparison of novel, passively dosed water accommodated fraction preparation methods and traditional low energy, and chemically enhanced, water accommodated fraction methods showed both methods to be applicable to laboratory testing. Finally, investigation of cardiotoxicity as a non-invasive, sublethal biomarker was investigated as an endpoint potentially more sensitive than the whole organism cyst based tests. The sensitivity of three life stages (pediveliger eyed larvae, spat and adults) of Pacific oyster (Crassostrea gigas) was assessed using a novel combination of passive dosing and non-invasive video graphic and infrared methods. Illustrating both the utility of the methods and relative insensitivity of the organisms to phenanthrene as a model compound. Owing to the lack of sensitivity observed in these studies, it was not possible to recommend the biological test methods used above as a complete rapid assessment toolkit. However, the combination of chemical and computational tools showed effectiveness in reaching and maintaining exposure concentrations and providing a conservative estimate of toxicity. A synthesis of findings concludes with suggestions for future directions for the development of oil spill monitoring tools; the consideration of multi-stressor impacts; recommendations of a screening matrix for the future identification of sensitive bioindicators; and alternative endpoints of interest (e.g. indicators of membrane damage, the aryl hydrocarbon receptor and indicators of oxidative stress).