The principal research question addressed in this thesis is how the effectiveness of an environmental legal system can best be evaluated. A legal system is effective if it is achieving or likely to achieve its objectives. For an environmental legal system this means achieving sustainable development. The hypothesis tested in relation to this research question is that the pressure-state-response ("PSR") method of State of the Environment ("SoE") Reporting provides the best available framework for evaluating the effectiveness of an environmental legal system. A subsidiary research question addressed in this thesis is whether the environmental legal system protecting the Great Barrier Reef ("GBR") in north-eastern Australia is likely to achieve sustainable development of it. The hypothesis tested in relation to this research question is that the environmental legal system protecting the GBR is likely to achieve sustainable development of the GBR. The principal method used to address these research questions and test the hypotheses is a case study of the effectiveness of the laws protecting the GBR. Particular emphasis is given in the case study to climate change both because it is now recognised as the major threat to the GBR and is a topic of significant international and national interest. This thesis is intended to contribute, in particular, to the current public and policy debate on responding effectively to climate change by using the GBR as a yardstick against which to measure "dangerous climate change" and, conversely, acceptable climate change. There are five major findings of the research. First, most of the legal writing regarding environmental legal systems is descriptive, explanatory and interpretative rather than evaluative. Second, most legal writers who attempt to evaluate the effectiveness of part or the whole of an environmental legal system implicitly use the PSR method and refer to pressures, conditions, and responses but do not acknowledge this conceptual framework. Third, the best available conceptual and analytical framework for evaluating the effectiveness of an environmental legal system is the PSR method. It is the simplest, most systematic, comprehensive and meaningful framework with the greatest predictive power for evaluating the effectiveness of the total social and legal response to human-induced environmental degradation currently available. Fourth, current practice in SoE reporting, at least in relation to the GBR, is largely descriptive and rarely evaluates the effectiveness of the response. The fifth major finding of this research is that, while there are many effective parts of the response to pressures on the GBR, the current environmental legal system is not likely to be effective in preventing climate change from causing very serious damage to the GBR. Based on what we know at this point in time, particularly the technology that is currently available and current greenhouse gas emissions, the impacts of climate change appear likely to swamp the many good aspects of the legal system protecting the GBR. Atmospheric concentrations of carbon dioxide in 2005 were approximately 379 parts per million ("ppm") and rising by 2 ppm per year. Including the effect of other greenhouse gases such as methane, the total concentration of atmospheric greenhouse gases was around 455 ppm carbon dioxide equivalents ("CO2-eq") in 2005, although the cooling effect of aerosols and landuse changes reduced the net effect to around 375 ppm CO2-eq. Limiting the total increase in mean global temperature to approximately 1°C requires stabilization of atmospheric greenhouse gases and aerosols around 350 ppm CO2-eq. Increasing the net effect of greenhouse gases and aerosols to 450-550 ppm CO2-eq is expected to result in a 2-3°C rise in mean surface temperatures. There are currently no international or national legal constraints to hold greenhouse gas concentrations beneath these levels and they appear likely to be exceeded. These increases in mean global temperatures are expected to severely degrade the GBR by 2030-2040. Even the targets being set by the new Australian Government of reducing Australia's greenhouse gas emissions by 60% by 2050 appear insufficient to protect the GBR. If a 60% reduction in emissions can be achieved globally by 2050 a rise in mean global temperature of around 2.4°C is expected. This indicates the environmental legal system protecting the GBR is not likely to be effective in relation to climate change and, therefore, is failing to reach its objective of sustainable development. Three major recommendations arise from the research. First, legal writers attempting to evaluate the effectiveness of the whole or part of an environmental legal system should use and acknowledge the PSR method. Second, SoE reports should include a stand-alone chapter evaluating the effectiveness of the response. Third, the environmental legal system protecting the GBR should take strong and comprehensive measures to reduce greenhouse gas emissions if the objective of sustainable development is to be achieved. Such measures should include setting policy targets for stabilizing atmospheric greenhouse gas and aerosol concentrations around 350 ppm CO2-eq to limit increases in mean global temperature to 1°C. Policy targets of stabilizing atmospheric greenhouse gases and aerosols at 450-550 ppm CO2-eq to limit increases in mean global temperatures to 2-3°C are likely to be too high to avoid severe impacts of coral bleaching to the GBR.