This chapter describes several conditions that have been associated with hypocapnia and in which capnography biofeedback may present a viable biobehavioral treatment option. Given the assumed central role of hypocapnia in panic development and maintenance, capnometry-assisted respiratory training (CART) was developed as a novel, non-pharmacological treatment to counteract the respiratory abnormalities observed in panic disorder (PD). To overcome the deficiencies of earlier hypoventilation training studies in asthma, CART has successfully been adapted to asthma patients. Panic and asthma patients in the studies described here showed clear clinical benefits from using a portable capnometer as a behavioral therapy tool. Portable capnometry devices facilitate the patient's home training efficiency, self-modification efforts, and treatment compliance by immediate, objective feedback of respiratory parameters. Ambulatory capnometry devices with electronic data storage allow the therapist to track a patient's progress without having to rely exclusively on retrospective self-reporting.