<p>Modal notions have been an intriguing topic in terms of capturing their crosslinguistic behaviors which have been analyzed as quantifiers (Hacquard, 2006; Kratzer, 1977), free choice items (Rullmann et al., 2008), or degrees (Lassiter, 2017). These typological patterns become more interesting when the simultaneous nature of sign languages has been added to the typology. By adding another dimension to the crosslinguistic patterns, sign languages have been reported to have different realizations for modals. Some of them have nonmanual markers alone for epistemic modals (Bross, 2018; Herrmann, 2013) while some have both manual signs and nonmanual markers (Karabüklü et al., 2018; Shaffer, 2004).</p> <p><br></p> <p>Bringing new data for the modal typology in spoken and sign languages, this dissertation analyzes the functions of modal signs and cooccurring nonmanual markers in Turkish Sign Language (TID). Even though manual signs and nonmanual markers appear together in modal sentences, nonmanual markers are shown to be neither lexical nor structural parts of modal signs. Manual signs are analyzed for their modal force and flavor with experimental studies. Results have shown that TID shows two typological patterns in its modal system: modals with specified modal force and flavor, and modals with specified force and unspecified flavor.  </p> <p><br></p> <p>One of manual signs, lazim ‘necessary’, along with epistemic signs were further investigated for their evidential requirement in epistemic contexts. Results showed that lazim requires a strong inference to be felicitous in epistemic contexts. Different than other languages, LAZIM in TID requires not only the right kind of context, but also the right morphological combination. It is interpreted as a deontic sign when it appears after verb by itself. In order to be interpreted as epistemic, it needs to appear after another sign ol which encodes the change of state.</p> <p><br></p> <p>Effects of nonmanual markers are investigated on perception of the signer’s certainty with an experimental study. Signer certainty is rated lower when the squint accompanies the sentence. In contrast, it is rated higher when head nod accompanies the sentence. The effect of increased perception of certainty with head nod is argued to result from the focus on the verb or the modal, yielding verum focus. Squint is analyzed as the uncertainty marker which can be anchored to the signer, the subject, or the addressee based on the structure in which it appears. Systematic analysis of nonmanual markers brings a new piece of evidence to the long-lasting discussion on where nonmanual markers function in sign languages’ grammars.</p>