Breathing problems are the root cause of the most common and serious of the sleep disorders. Respiratory dysfunction can manifest only during sleep or can be present in wakefulness and made worse by sleep. This article identifies foundational concepts and principles of the control of breathing to explain why the respiratory system is vulnerable in sleep. The relation of these foundational concepts and principles to the spectrum of clinical sleep-related breathing problems are identified. Ultimately, understanding such mechanisms is the basis of rational strategies to improve breathing in sleep and the clinical course.