A very large proportion of non-Hodgkin's lymphoma in the United States are of B-cell origin. This group of tumors includes a variety of different pathological and clinical types. Chromosomal rearrangements play an important role in the pathogenesis of many of these tumors. In B-cells these translocation processes appear to develop as illegitimate products of physiological V-(D)-J or heavy chain switch rearrangements. The biology of the well-known chromosomal translocations is discussed. Additional biological factors in lymphomagenesis (aging, immunodeficiency, role of antigenic stimulation, and genetically determined susceptibility) are discussed.