Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a complex, chronic, debilitating disease with systemic effects. ME/CFS is characterized by reduced ability to perform pre-illness activities that lasts for more than 6 months and is accompanied by profound fatigue, which is not improved by rest. A hallmark of ME/CFS is that symptoms can worsen after physical, mental, or emotional effort, a manifestation known as post-exertional malaise (PEM). Patients with ME/CFS also have unrefreshing sleep. Other common manifestations are orthostatic intolerance, cognitive impairment, and pain. As can be observed in people with other long-term chronic illnesses, secondary psychological symptoms such as depression and anxiety may also be present in some patients with ME/CFS.