Injury to the right phrenic nerve may range from transient impairment of diaphragmatic function to permanent phrenic nerve palsy (PNP). The phrenic nerve is separated from the superior vena cava (SVC) by only the pericardium at the anterolateral junction between the SVC and the right atrium. During the cryoenergy application, multiple modalities are currently utilized to monitor phrenic nerve function while pacing the nerve from the SVC. Phrenic nerve injury (PNI) is a complication associated with cryoballoon ablation that can be avoided with appropriate planning and monitoring. Early detection of PNI and immediate termination of ablation are essential in the prevention of PNP. This chapter, describes the methods for detection of PNI and it is imperative to stop ablation immediately. PNI is the most common complication associated with circumferential ablation of the pulmonary veins using cryoballoon catheters to treat atrial fibrillation.