Since spectrum is a precious commodity, knowing how efficiently different technologies and applications use it allows users and operators to make the best decisions on what wireless technology to deploy and in what configuration. Understanding spectrum efficiency can also assist policy makers better decide on how to most effectively allocate spectrum and to whom. Unfortunately, multiple methods exist to measure the efficiency of spectrum use, each with its advantages and disadvantages, and no single measure works for all scenarios. Nevertheless, knowing these measures and their characteristics can lead to wiser decisions about spectrum allocation and deployment.