For the hematopathologist involved in scientific research, lymphomas and leukemias present the most challenging field of medical investigation, one in which immunity and neoplasia interface and in which new concepts and techniques are constantly tested. For the practicing pathologist, the important task of correctly diagnosing and classifying leukemias and lymphomas arises frequently and can be difficult. To establish the treatment and determine the prognosis, Hodgkin and non-Hodgkin lymphomas must be identified and phenotyped. Non-Hodgkin lymphomas must be differentiated from Hodgkin lymphomas, and both must be separated from the various types of non-neoplastic lymphadenopathies. Once identified, they must be characterized morphologically, immunologically, and genotypically. An array of diagnostic methods and a wide variety of reagents and techniques, unmatched by those in any other area of diagnostic pathology, are available. It is incumbent on the practicing pathologist to set up standards and select methods that are most likely to facilitate the proper diagnosis at a reasonable cost.