Index decomposition methodology was a technique first used in the late 1970s to study the impact of changes in product mix on industrial energy demand. A survey in 1995 listed a total of 51 studies. Since then, many new studies and several new decomposition methods have been reported and the methodology has been increasingly used in energy-related environmental analysis. We trace these new developments, discuss method formulation using an index number framework, and classify more than one hundred studies based on application area, aggregate indicator, and decomposition scheme. Application issues useful to researchers undertaking new studies and possible areas for future research are presented.