End-tidal CO2 (ETCO2) monitoring is not a new modality in the pediatric emergency department (PED) and emergency department. It is the standard of care during certain procedures such as intubations and sedations and can be used in variety of clinical situations. However, ETCO2 may be underused in the PED setting. The implementation of ETCO2 monitoring may be accomplished many ways, but a foundation of capnography principles specifically in ventilation, cardiac output, and current literature regarding its application is essential to successful implementation. It is the intention of this article to briefly review the principles of ETCO2 monitoring and its clinical applications in the PED setting.