This chapter focuses on the neuroimaging of the brainstem in parasomnia overlap disorder using a case of a 40-year-old woman who developed acute rhombencephalitis with cerebellar ataxia, a facial palsy, diplopia, right hearing loss, dysphagia, and dysarthria, as an example. One year after the first neurological episode, she underwent video-polysomnography. The video-polysomnography showed rapid eye movement (REM) sleep that lasted 90 min. On initial magnetic resonance imaging (MRI), there were gadolinium-enhanced hyperintense signals on the T2-weighted and fluid-attenuated inversion recovery (FLAIR) sequences and hypointense signals on the T1-weighted sequences in the mesencephalic tegmentum, fourth ventricle floor, and right medulla, suggesting diffuse inflammatory lesions. Brainstem lesions have been reported previously in association with parasomnia overlap disorder. This report provides evidence that lesions within or near the pontine tegmentum are associated with REM sleep behavior disorder (RBD) and parasomnia overlap disorder and that a unilateral lesion by itself cause RBD and sleepwalking.