Sustainability assessment has emerged as a distinct form of impact assessment. It is a process which commences before an action is taken in order to inform decision-makers about the potential future consequences of such development and elicit appropriate responses. Sustainability assessment retains many characteristics of the techno-rational roots of impact assessment as a governance mechanism. This chapter puts forward two emerging methodological approaches which have the potential to overcome some of the limitations of the currently prevailing practice and greatly strengthen the application of sustainability assessment as a governance mechanism for sustainability. They are: systems analysis and deliberative approaches, to be applied within an eight-step process, covering: decision to conduct a sustainability assessment; identification of the desired outcome and hence the decision question; establishment of sustainability goals and criteria; identification of alternatives and option to achieve the outcome; prediction and evaluation of the impacts of each alternative; selection and enhancement of the preferred option; approval decision and announcement; and implementation, monitoring and follow-up.