Nowadays, sustainable development is one of the most important challenges to be confronted by all countries on the globe. The aim is to obtain economic increase without causing damage to the natural capital and maintain peoples well being. In order to diagnose the development of the countries, many tools with different approaches can be used, all of them supplying performance indices as a final result. However, indices and indicators have been misused in most published works, and as a result, there has been an incoherent discussion about their true meaning. In this way, the present work discusses and tries to clarify the meaning of words associated with sustainability, considering as examples three indices that are much used to diagnose the development of the countries: Environmental sustainability index; Ecological footprint; Emergy performance index. © 2008 ANPPAS - UNICAMP.