This article reviews several recent studies suggesting that — contrary to a widespread belief — adult monolingual native speakers of the same language do not share the same mental grammar. The studies examined various aspects of linguistic knowledge, including inflectional morphology, passives, quantifiers, and more complex constructions with subordinate clauses. The findings suggest that, in some cases, language learners attend to different cues in the input and end up with different grammars; in others, some speakers extract only fairly specific, ‘local’ generalizations which apply to particular subclasses of items while others acquire more abstract rules which apply ‘across the board’. At least some of these differences are education-related: more educated speakers appear to acquire more general rules, possibly as a result of more varied linguistic experience. These findings have interesting consequences for research on bilingualism, particularly for research on ultimate attainment in second language acquisition, as well as important methodological implications for all language sciences.