To clarify the clinical manifestation and differential diagnosis of spasmodic dysphonia, the recent literature is reviewed with special regard to “Diagnostic Criteria and Severity Classification for Spasmodic Dysphonia,” which was recently published by a research group using a Health and Labor Sciences Research Grant. Spasmodic dysphonia is a focal movement disorder of the larynx. The main symptom of spasmodic dysphonia is intermittent abnormality of phonation mixed with normal voice production during speech. The voice property is represented as strangulation, interruption, tremor or roughness in adductor type and breathiness, aphonia or flutter in abductor type. The symptoms of spasmodic dysphonia are well explained as an expression of focal dystonia of the larynx. Differential diagnoses should be made from dysphonia of other functional, psychosomatic and neurological origins. Precise history taking, a psychoacoustic evaluation and an acoustic analysis of the speech using symptom-inducing tasks and endoscopic observation during speech are mandatory. Diagnostic intervention with voice therapy or botulinum toxin injection into the internal laryngeal muscles is helpful for managing difficult cases.