This article reports the results of a study of necessity and possibility modals in Brazilian Sign Language (Libras). The data discussed here mainly come from an interview carried out with a fluent deaf Libras signer. Our analysis suggests that the Libras modals we elicited have a lexical source with a more concrete meaning (Bybee 1994 et al.) and, in some cases, can have their origin traced back to a gesture, as has been demonstrated for ASL (Wilcox & Shaffer 2006). It also suggests, based on synchronic polysemy, that modals in Libras have developed through the same grammaticization processes observed for spoken languages. 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