Superior semicircular canal dehiscence (SCD) can be defined as both an anatomic entity and a clinical syndrome within the larger category of Third Mobile Window Disorders. The anatomic defect in SCD may be a developmental anomaly of the temporal bone that is typically asymptomatic until a second event later in life. However, some have posited the theory that gradual thinning of overlying bone results in an acquired SCD. The second event, either trauma or a pressure-altering event, usually precedes the onset of the clinical syndrome of SCD. SCD symptomatology can be quite diverse with multiple presentations, mimicking other otologic conditions such as Meniere’s disease, patulous eustachian tube, and otosclerosis. The most espoused theory for the symptom complex has been the Third Mobile Window theory. However, there are several observations not fully explained by this theory. An alternate theory that explains these observations is a theory espousing changes in the round or oval window compliance with or without concomitant perilymphatic fistula (PLF). The diagnosis of SCD requires three elements: (1) history compatible with SCD, (2) physiologic testing compatible with SCD, and (3) high-resolution CT scan demonstrating SCD. While these three elements are required to make the diagnosis of SCD, the clinician is encouraged to do a full battery of audiovestibular tests in these complex patients. Many will have abnormalities on vestibular testing that may not be expected and could have a significant impact on treatment/counseling of the patient as well as long-term management. This chapter will discuss the pathophysiology and diagnosis of SCD.