The spectrum of the histopathology of Langerhans cell histiocytosis was presented with special attention paid to the pathologic Langerhans cell as the diagnostic lesional cell. Features that distinguish this histiocyte from others are defined by electron microscopic demonstration of Birbeck granules and by immunohistochemical characteristics such as possession of antigenic determinant(s) that react with the monoclonal antibody T6. The relationship between Langerhans cells and the other histiocytes from which they are derived seems to be determined by events initiated by surface antigens reacting with specific antibodies. Immunohistochemical tests that are of value in diagnosing and classifying the various histiocytosis syndromes include those used to identify the S-100 antigen, T6 antigenic determinant(s), and peanut agglutinin binding sites. The use of a standard nomenclature and diagnostic criteria for the histiocytosis syndromes is encouraged as a step forward in progress to better understand and communicate about these puzzling disorders.