CD40 is a molecule in the tumor necrosis factor receptor/nerve growth factor receptor (TNFR/NGFR) family that is present on both normal and neoplastic B lineage cells. It is also expressed on carcinoma and melanoma cells and can be augmented with interferon gamma. CD40 stimulation in normal B cells has been demonstrated to promote normal B cell differentiation and growth in vitro. In contrast to these effects, CD40 stimulation by either anti-CD40 antibodies or a recombinant soluble CD40 ligand can inhibit the growth of human breast carcinomas and aggressive histology B lymphomas in vitro and in vivo. This is believed to occur by activation-induced cell death (AICD) in which stimuli that promote the growth of normal cell types inhibit the growth of neoplastic counterparts. This occurs through the induction of apoptosis, necrosis and/or cell cycle arrest. Thus, CD40 stimulation may be of potential clinical use in the treatment of carcinomas and B cell lymphomas. This review shall provide an overview of the various effects of CD40 stimulation on both normal and neoplastic cell types.