Heidegger's concepts of `discourse' and 'language' play a fundamental role in the interpretations of his early work, especially in Being and time. Nevertheless, the exact extent of their functions differs between interpretations and scholars. This multiplicity of interpretations is a direct answer to the constitution of language itself, as an hermeneutical phenomenon. But even though this existential concept of language can be approached from different points of view, its meaning is ultimately decided by the internal coherence of Heidegger's philosophical project and by the task of the existential analytic as a whole. This paper attempts to offer a general description of the constitution of the concept of language in Being and time; and also to provide an account of language's ontological horizon: that is, the fundamental problem of possibility. Thus, by attending to the complex set of relations that determines the organization of the concept of language we will be able to read Heidegger's Being and time in a new way.