The Language Endangerment Index (LEI) is a tool developed by Endangered Languages Catalog (ELCat) researchers for assessing and quantifying how endangered a language is. This chapter explains how the LEI was developed and how to use it, with reference to specific languages as examples. LEI comprises four individual scales, each concerning a factor associated with language loss: intergenerational transmission, absolute number of speakers, speaker number trends and domains of use. LEI was devised with the successes and shortcomings of other methods of vitality assessments in mind. The LEI, designed as a central part of ELCat, has given the field a finely tuned tool for assessing the level of endangerment of languages all over the world. A language that is considered to be "Critically Endangered" on the scale of intergenerational transmission has only a few elderly speakers, and no one else speaks the language.