This work is a comprehensive review of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) studies that were conducted with both community-based and hospital-based samples. A review of the prevalence of fatiguing illnesses in varied countries of the world shows evidence that the rates of fatigue and its syndromes vary across settings and countries, and that the methodology used impacts findings. Studies have also shown the presence of a severe and disabling form of fatigue that affects the ability of individuals to engage in normal occupational, educational, social and personal daily activities. The history, definition and research findings in ME/CFS are presented to promote an understanding of the work that has been accomplished in this research area in varied continents of the world. It is hoped that some of the issues addressed in this work will help people to better understand ME/CFS and fatiguing illnesses reported in varied countries. Perhaps, the understanding that ME/CFS is a global condition might encourage both scientists and practitioners to work towards streamlining the definition and diagnostic criteria to strengthen work in this area and ultimately improve the treatment of persons with ME/CFS.