End-tidal carbon dioxide monitoring (ETCO2) has clinical uses far beyond solely determining hypo- or hyperventilation. This non-invasive monitor can give valuable information about cardiac output, perfusion, and ventilation. Capnography has become one of the mainstays of anesthetic monitoring today. This lecture is designed to provide the attendee with an overview of the technology and concept of PaO2 vs ETCO2, and to provide detailed explanations and interpretations of a variety of capnograms – the accompanying waveform of many capnography analyzers.