Obstructive sleep apnea (OSA) is caused by, among other things, hypotonia and dysfunction of the orofacial and pharyngeal muscles and is related to an increase in comorbidities in patients who suffer from it. Its main treatment is night therapy with continuous positive airway pressure (CPAP), but sometimes it becomes necessary to look for alternative therapies. To make a suitable selection of the applicants, it is necessary to conduct an examination that allows for an appropriate view of the location and pattern of collapse in the upper airway. This is performed by drug-induced sleep endoscopy (DISE), in which an endoscopic exploration is conducted while sleep is simulated pharmacologically. In this chapter, we are going to explain what this exploration consists of and its relation to myofunctional therapy. Although DISE shows which structures produce the collapse of the upper airway, it does not allow us to differentiate whether there is a myofunctional disorder or not. Nevertheless, we know that myofunctional disorders have an influence on OSA and, therefore, on the results of the DISE. Some aspects to evaluate are, for example, tongue tone or epiglottic collapse, both of which can improve with myofunctional therapy. It is also important to keep in mind other aspects, such as correct lip sealing or palate structure, and their relation to upper airway collapsibility.