The biblical story of the shibboleth is widely cited in language testing as emblematic of the social and political function of language tests. But the meaning of the shibboleth has also been explored within poststructuralism, specifically within Derrida's discussion of the dilemmas of identity in the work of the German Jewish poet Paul Celan. Derrida discusses language itself as shibboleth, and emphasizes the ambiguity and indeterminacy of the linguistic sign, its "undecidability". This article considers the implications of Derrida's argument for the area of language testing in order to see what his interpretation of the shibboleth might mean for understanding its practices, typically framed as they are within a modernist paradigm. Examples are drawn from various areas of language assessment, including the use of language analysis in the determination of the claims of asylum seekers. What implications for understanding such assessment practices does a poststructuralist perspective offer?