Evidentiality is referred to as the linguistic expression of information sources marking how the speaker knows about an event. In Turkish, expressing past events has two ‘flavors’: the direct (-DI) or the indirect evidential (-mIş) forms have to be suffixed on the verb depending on whether the event is known through direct or indirect information sources, respectively. This dissertation aims to unveil the extent to which the evidentiality system in Turkish is susceptible to types of language loss induced either by agrammatic aphasia (a language disorder due to brain damage) or by heritage language acquisition in early bilinguals. Our findings have shown that evidentiality is affected in both individuals with agrammatic aphasia and in heritage speakers of Turkish. Although underlying mechanisms involved in these impairments and attrition effects are different, the outcomes are identical: the direct evidential form is the most affected in both pathological and non-pathological language loss. Clearly, the agrammatic speakers’ difficulty in the direct evidential is the assignment of past time reference; a link between speech time and witnessed event has to be established, which is computationally hard for agrammatic speakers to process. The heritage speakers, patterning with late bilingual speaker, have been shown not to process the direct evidential form with a native-like sensitivity as confirmed by an eye-movement monitoring experiment. Furthermore, the heritage speakers are insensitive to violations in information source contexts by both direct and indirect evidential forms. The evdientiality paradigm in Turkish heritage grammars is concluded to have undergone restructuring and simplification in its semantic and pragmatic contents.