This chapter reviews the usefulness of Meta-iodobenzylguanidine (123I-MIBG) in neurology-specific sleep disorders with a particular focus on rapid eye movement (REM) sleep behavior disorder (RBD). Several studies demonstrated the diagnostic reliability of reduced cardiac 123I-MIBG uptake in Parkinson's disease (PD) patients. In general, 123I-MIBG scintigraphy is regarded as an assessment of autonomic function. Autonomic dysfunction is common in α-synucleinopathies, and particularly marked in multiple system atrophy (MSA). Most patients with idiopathic RBD (iRBD) have reduced 123I-MIBG uptake, and an abnormal 123I-MIBG finding supports the diagnosis of RBD. Although an abnormal 123I-MIBG finding cannot predict the development of α-synucleinopathies among iRBD patients, the existence of RBD might be predictive of developing PD with dementia (PDD) in patients with PD. In the near future, 123I-MIBG findings might help predict the development of PDD among PD patients affected with RBD.