The lack of familiarity of sociolinguistics with social theory, leads to use indifferently, as if they were transparent and univocal, items that present problems since the beginning of their use as technical items. This is the case of the term identity. From the modern perspective, theories on identity, formulated by social sciences, can be ascribed to two main groups: essentialists and constructionists. The perspective of this paper belongs to the latter. Because of its relevance, Tajfel´s theory of social identity stands out. This theory has been recently developed by Hogg through the theory of self-categorization. This work deals with the relationship that languages can maintain with identities under the light of these theories. Languages and linguistic varieties are, on the one hand, the result of social identity acts, and, on the other hand, the source of social identities (among which ethnic and national identities are found, but not exclusively). Likewise, reflexive modernization theory is based on ”the fact that social practices are constantly examinated and improved under the light of the information that is extracted from those practices, that, in this sense, alter their characteristics” (Giddens). All this process affects identities and on their relations to languages and varieties.