Langerhans cell histiocytosis (LCH) has presented problems in diagnosis and in treatment. The definitive diagnosis is a pathologic one. To advance thorough and uniform assessment of these patients, the elements of a complete clinical and laboratory evaluation are presented by the Histiocyte Society. Adoption of a uniform clinical/laboratory approach to these children, coupled with agreed-upon steps to the pathologic identification of LCH, should result in a better understanding and ultimately better and more rational therapies for the disease.