Sustainability is rapidly moving from an abstract concept to a measurable state of dynamic human-ecological systems. The large number of economic, social, and environmental indicators currently available provides an unwieldy view of system sustainability. To aid policy decisions, these indicators are therefore either presented in the context of a conceptual framework, or quantitatively aggregated into indices. Due to the quantitative nature of sustainability indices, their results may be given more weight by scientists and policy-makers. However, policy decisions can be ineffective or even counterproductive if they do not consider factors which influence index behavior: the scale of the available data and choice of system boundaries; the inclusion, transformation, and weighting of indicator data; and the aggregation method used. As previous reviews have demonstrated, sustainability indices do not rank countries consistently, even when using some of the same indicator data. Several improvements would increase the utility of sustainability indices for policy decisions, particularly the identification of biases introduced by the index methodology and data inclusion. The discrepancy of current sustainability indices due to theoretical or methodological differences supports the use of several complementary indices.