Hodgkin's disease (HD) is characterized by the presence of the typical, clonal malignant Hodgkin and Reed-Sternberg (H-RS) cells in a hyperplastic background of normal reactive lymphocytes, plasma cells, histiocytes, neutrophils, eosinophils and stromal cells. The neoplastic nature of HD is based on aggressive clinical progression, presence of the proliferating and atypical H-RS cells, aneuploidy and cellular clonality. Immunophenotypical studies have demonstrated frequent expression of lymphoid "activation markers' including CD15, CD25, CD30, CD40, CD54, CD70, CD71, CD80, CD86 and MHC class II and less frequent expression of T- or B-cell-associated antigens by the neoplastic H-RS cells. The clonality of H-RS cells is demonstrated by clonal EBV integration, clonal cytogenetic abnormalities including p53 mutations and clonal immunoglobulin rearrangements in some HD cases. There is involvement of diverse molecules with oncogenic potential, including presence of viruses (Epstein-Barr virus and human herpes virus-6) and/or oncogenes/tumour suppressor genes (bcl-2/bcl-x, p53/MDM-2, c-myc, c-fms, N-ras, lck). The histopathological presentation and characteristic clinical features of HD correlate with an unbalanced production of multiple cytokines and define HD as a tumour of cytokine-producing cells. The proportion of malignant H-RS cells to reactive cellular components and fibrosis is dependent on the production of particular cytokines and allows subtyping of HD cases. The combined use of immunohistochemical, biochemical and molecular techniques has thus allowed recognition that HD represents more than one clinico-pathological entity with different types of H-RS cells. The defined mechanism for the biological nature, origin and oncogenesis of H-RS cells remains not fully understood, but is susceptible to further analysis using modern technology.