Sleep apnea or sleep apnoea is a sleep disorder characterised by pauses in breathing during sleep. These episodes, called apneas (literally, "without breath"), each last long enough so one or more breaths are missed, and occur repeatedly throughout sleep. There are two distinct forms of sleep apnea: Central and Obstructive. Breathing is interrupted by the lack of effort in Central Sleep Apnea, but from a physical block to airflow despite effort in Obstructive Sleep Apnea. In Mixed Sleep Apnea, both types of events occur. Regardless of type, the individual affected with sleep apnea is rarely (if ever) aware of having difficulty breathing, even upon awakening. Sleep apnea is recognised as a problem by others witnessing the individual during episodes, or is suspected because of its effects on the body (sequelae). This volume examines leading-edge research important for an understanding of the field.