Since history and physical examination alone cannot reliably diagnose obstructive sleep apnea (OSA), the gold standard for the diagnosis of OSA is polysomnography. When an oral device or surgery is considered, it is of utmost importance to examine an individual's pattern, degree and site(s) of upper airway obstruction. This article tries to evaluate recent literature published on the use of (drug-induced) sleep endoscopy in evaluating the individually tailored treatment. Different techniques, interrater reliability, test-retest reliability and currently available data on the relationship with treatment outcome are reviewed.