The resemblance between obese sleepy patients and Joe, the sleepy character in Dickens’s book The Pickwickian Papers was first alluded to in the medical literature toward the end of the 19th century. Monitoring respiration in Pickwickian patients during sleep uncovered repeated apneic episodes causing intermittent hypoxia and sleep fragmentation which led to the description of OSA. Since then, it has been established that despite similar presenting symptoms and patterns of sleep-disordered breathing, some Pickwickian patients also have chronic daytime hypoventilation. These patients are designated as having obesity-hypoventilation syndrome (OHS) that is distinctly different than OSA. OHS is defined by the presence of daytime hypercapnia (PAco2 ≥ 45 mm Hg), sleep-disordered breathing, and obesity (BMI ≥ 30 kg/m2), after exclusion of all other possible causes of chronic hypercapnia. However, the diagnostic criteria of OHS, particularly with respect to the necessity of sleep-disordered breathing and hypercapnia, are still controversial.