Cardiopulmonary coupling (CPC) refers to the relationship between the cardiovascular and respiratory systems. Cardiopulmonary synchronization is a physiological phenomenon observed during sleep. The synchronization intensity of heart rate variability (HRV) and respiration allows the measurement of sleep characteristics such as sleep stability and fragmentation. In this chapter, the application of CPC in identifying obstructive sleep apnea will be reviewed. Using Fourier transformation, cross-spectral power and coherence of HRV and respiration signals are calculated, and further used to calculate the ratio of coherent cross-power in different frequencies. The low-frequency coupling (LFC) reflects cyclic pattern across multiple breathing cycles and is associated with cyclic alternating pattern in electroencephalogram. LFC is often linked to disturbed sleep or fragmented sleep. Elevated LFC coupling (e-LFC) is identified apnea/hypopneas has been defined as a minimum low-frequency power > 0.05 normalized units with a low-to-high frequency ratio >30 (Seo et al. Clin Exp Otorhinolaryngol 14:424–6, 2021). Third, very-low-frequency coupling occurs in rapid eye movement (REM) sleep and wakefulness. The logarithm of high-to-low frequency CPC ratio is often used for measuring continuous varying CPC. Patients with obstructive sleep apnea (OSA) have increased LFC, and continuous positive airway pressure (CPAP) therapy decreases LFC. Simple CPC showed a high correlation with AHI from polysomnography (PSG). CPC data combined with oxygen desaturation to calculate AHI has been approved by the USA Food and Drug Administration for diagnosis of OSA. Notably, AHI from CPC was lower than PSG in pediatric patients with non-severe OSA.