This chapter focuses on the importance of the potential role of capnography during sleep. It provides a clear definition of the unique changes in ventilation that accompany the sleep state and its various components. With the evolution of significant clinical interest in sleep-related breathing disorders, the use of polysomnography has been expanded to include more detailed assessment of breathing during sleep. Capnometry is the measurement of CO2 concentration in a gas mixture denoted by a continuous waveform display. Sleep serves multiple functions in humans, including biochemical (anabolic hormone secretion, protein synthesis, energy conservation), physiologic, and neurological. It is extensive in its monitoring capacity, but retains significant limitations in its capability to evaluate changes in ventilation and breathing outside of frank obstructive sleep apnea (OSA) for which it was designed. Comprehensive incorporation of capnography into clinical practice has great potential for enhancing the sleep evaluation in many patients.