Catathrenia is a peculiar sleep-related phenomenon that is associated with the production of loud groaning sounds during episodes of prolonged expiration. The first case was reported in 1983 by De Roeck et al. It has a typical respiratory pattern of bradypnoea consisting of a deep inspiration followed by a long expiration and noiseresembling central apnoeas. The analysis of the harmonics and formants showed that catathrenia has a vocal origin with a predominant frequency. Although there is consensus about the above mentioned characteristics, there are controversies about the classification among the sleep disorders. In fact, there is no full agreement about its nature, origin, meaning, and treatment. The International Classification of Sleep Disorders-Second Edition (ICSD-2) classified catathrenia among parasomnias; however, the ICSD-3 includes it among the respiratory disorders (isolated symptom or normal variant inside the respiratory disorders of sleep). Supporting its classification within the parasomnias, it can be considered a complex respiratory motor behavior consisting of a deep inspiration, followed by breath holding and slow release of air through a closed glottis, during which the groaning sound is produced. Furthermore, it is often associated with spontaneous arousals that may precede or coincide with the vocalization. On the other hand, it can be considered a breathing related disorder because, as stated in the ICSD-3, it also has a respiratory pattern of bradypnoea consisting of a deep inspiration followed by a long expiration and noiseresembling central apnoeas, although is not usually accompanied by evident oxygen desaturation. The heterogeneity in the published cases could be the origin of the discrepancies. According to the duration of the sound there may be at least two different types of patients: in some of them the duration is short (0.5–1.5 s) and in others it is longer (2–50 s); although long and more pronounced bradypnoeic events seem to occur exclusively during REM sleep and the index of bradypnoeic events increase with consecutive REM cycles, some reports have indicated that the sound could appear at any time during the night, or even with some predominance during non-REM stages. The intensity of the sound also has a great variability between 40 and 120 dB. Globally, patients with the long sound and with respiratory dysrhythmia might be different from patients with the short and very intense sound. In conclusion, the questions about the nature of catathrenia, its classification and the decision if it is a single phenomenon or if it comprises two or more variants or disorders are still opened.