Attempts to understand the malignant lymphomas have followed two major lines of investigation: attempts to define the relationship of neoplastic lymphoid cells to stages of normal T- and B-lymphocyte differentiation, and attempts to understand the genetic abnormalities associated with malignant transformation. It is hoped that these two lines of investigation will make it possible to define clinically important entities, so that specific treatment strategies can be developed and tested, and will also point the way to new options for treatment and prevention. Despite the advances in immunology of the last two decades, our understanding of the relationships of lymphoma cells to the normal immune system remains incomplete. Techniques of molecular genetics, which initially promised to resolve questions of clonality and lineage, have proven less definitive than had been hoped. Studies in the field of molecular cytogenetics--the study of chromosomal translocations by molecular techniques--have just begun to unravel some of the mysteries surrounding neoplastic transformation and differentiation. As we enter the last decade of the century, small advances continue to chip away at the uncertainties shrouding the malignant lymphomas. Advances in the past year include the subclassification of low-grade B-cell lymphomas, including so-called intermediate lymphocytic lymphoma and extranodal lymphomas of mucosa-associated lymphoid tissue, the classification of T-cell lymphomas, the definition of Hodgkin's disease and its relationship to anaplastic large cell lymphoma, and new techniques for characterizing lymphoid cells in paraffin-embedded tissue sections.