This primer has been written for the clinical practitioner. Our goal is to provide the information necessary to understand, diagnose, and manage the symptoms of chronic fatigue syndrome — also known as myalgic en-cephalomyelitis (ME/CFS). The authors have made considerable efforts to ensure that the information provided is accurate and up to date. Since the extant literature does not adequately describe the nature and treatment of this illness, this document is written as a primer and is not “clinical practice guidelines” as recently redefined. Where published studies are lacking, our recommendations are based on the clinical expertise of our experienced practi¬tioners. Our hope is that you find the primer to be a useful adjunct to your practice and a worthy companion to your reference library.