This research empirically examined the relationship between evidentiality and modality in sentence interpretation by Turkish vs. English speakers and the influence of different forms of evidential marking on the establishment of discourse coherence. Evidentiality, a property, commonly refers to the linguistic marking (in the grammar or the lexicon) of source of knowledge about an asserted event. What is unclear is whether this property also conveys epistemic value (or stance information). This research examined this issue by speakers of Turkish (in which evidentiality is marked in the grammar) and English (in which evidentiality is marked in the lexicon). In Experiment 1 participants were presented with identical sentences differing only in whether evidential or modal markers were inserted. For each sentence they were asked to make judgments about the source of evidence and about their relative confidence about whether the asserted event had actually occurred. The results demonstrated that both Turkish and English speakers found that there was enough information to judge the source and degree of certainty of various evidential and modal expressions. The results support the view that there is a close relationship between evidentiality and modality. Further, it was found that the linguistic level of evidentiality indication affected the source and epistemic value interpretations. Evidential expressions were interpreted in more varied ways by Turkish speakers, while modal expressions were interpreted in more varied ways by English speakers. The second experiment used a discourse completion task in which participants read two sentences containing different evidential expressions that resulted in somewhat contradictory information. Participants were asked to supply a third sentence that would make sense of the first two. Along with the evidentiality manipulation, two other variables were manipulated: whether the evidential information was presented first or second and whether the asserted facts were general or particular. The results suggested that evidentiality marking affected speakers' sense-making process but was not the only factor influencing their response, since presentation order and the type of information (general vs. particular) also mattered. Interestingly, Turkish speakers appeared to place more emphasis on the nature of the fact in arriving at their response, whereas English speakers were more influenced by the order of presentation of the information. Taken together, the findings suggest that evidentiality conveys epistemic value of the reported event along with source of knowledge. Further, evidentiality - in interaction with contextual factors -- influences speakers' attempts at establishing discourse coherence. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/152717