The abnormal muscle recording (AMR) is a tool that can aid in the diagnosis of hemifacial spasm (HFS) and predict successful outcomes during microvascular decompression (MVD). Vascular compression of the facial nerve is the usual cause for HFS, and although there is no clear consensus as to its etiopathogenesis, this compression is thought to create abnormally low thresholds of excitation (a peripheral etiology) and/or facial neuron hyperexcitability (a central etiology). Both mechanisms likely contribute to generating the AMR. The AMR is an abnormal electromyography (EMG) recording made from a facial muscle when a branch of the facial nerve is stimulated that typically does not innervate that target muscle and reflects the abnormal spread of excitation observed in patients with HFS. This response, monitored intraoperatively, is expected to disappear in a patient who has undergone a successful microvascular decompression of the seventh cranial nerve. This chapter includes details on the history of the AMR, electrophysiology and pathophysiology of HFS, endoscopic MVD for HFS, the value of the AMR for predicting clinical outcomes, and alternative monitoring modalities, along with techniques for eliciting the AMR, troubleshooting methods, and anesthetic considerations.