Sustainability is often poorly defined and difficult to measure. We describe several concepts from ecology, economics, and physics, that have contributed to sustainability indices, and discuss their positive and negative aspects. Indices range from mostly ecological (such as ecosystem resilience and global human carrying capacity), to those inspired by both economics and ecology (green income and maximum sustainable yield), to a mix of ecology and physics (exergy and emergy). Economic concepts such as substitutability of natural and human capital (the “weak” versus “strong” sustainability debate), and through put of natural resources through an economic system, are the basis for several strictly economic indices. The second law of thermodynamics, which dictates the decrease in usable energy, has also had an increasing influence on sustainability discussions. The indices described here address different aspects of the interactions between human societies and ecosystems, and are therefore probably most effective when used in combination.