Immunoglobulin (Ig) gene remodeling by V(D)J recombination plays a central role in the generation of normal B cells, and somatic hypermutation and class switching of Ig genes are key processes during antigen-driven B cell differentiation. However, errors of these processes are involved in the development of B cell lymphomas. Ig locus-associated translocations of proto-oncogenes are a hallmark of many B cell malignancies. Additional transforming events include inactivating mutations in various tumor suppressor genes, and also latent infection of B cells with viruses, such as Epstein-Barr virus. Many B cell lymphomas require B cell antigen receptor expression, and in several instances chronic antigenic stimulation plays a role in sustaining tumor growth. Often, survival and proliferation signals provided by other cells in the microenvironment are a further critical factor in lymphoma development and pathophysiology. Many B cell malignancies derive from germinal center B cells, most likely because of the high proliferation rate of these cells and the high activity of mutagenic processes.