Polysomnography(PSG) is essential for the diagnosis of all pathophysiologic conditions due to both any types of sleep disordered breathing and to any types of sleep disturbance. Since this issue's objection is to describe the outline of PSG, we will explain here about when PSG is needed, what type of measurements are necessary for PSG, and how it should be interpreted from the result of the PSG tracing for the diagnosis for sleep apnea syndrome, etc.