This chapter illustrates the various ways in which languages differ from each other and then raises the question in what sense one can talk about universals despite this apparent diversity and variation. It reviews the extent of the diversity that we know from today's languages in the world, by highlighting the core domains of language: phonology, morphology, syntax, and semantics. In the chapter, the author uses the term language in the sense of a structure identifier, and applies it to any kind of variant set of structures, including dialects, sociolects, and idiolects. The range and nature of absolute universals depends entirely on the nature of the analysis and the descriptive metalanguage that one uses for a particular phenomenon. The criterion of historical realism is foundational for research on statistical as opposed to absolute universals because statistical universals are fundamentally historical in nature.