The B cell antigen receptor (BCR) is composed of the membrane form of the immunoglobulin (Ig) and the Ig-alpha/Ig-beta heterodimer, which function as the antigen recognition component and the signaling component, respectively. A signal transmitted by BCR modulates gene expression, adhesion or survival, thereby determining the fate of antigen-encountered B cells. BCR proximal signaling occurs within cholesterol- and sphingolipid-rich plasma membrane microdomains termed lipid rafts, and involves tyrosine kinases such as Lyn, Syk and Btk and the adapter molecule SLP65/BLNK. Although the distal signaling cascades via BCR are not yet fully elucidated, various components are already identified, such as lipid kinases and small G-proteins. BCR signaling is regulated by various membrane molecules termed co-receptors such as CD19 and CD22. The BCR co-receptors appear to be required for normal immune functions. Viral proteins such as LMP2 also regulate BCR signaling to maintain viral latency. Various aspects of BCR signaling and its regulatory mechanisms are discussed in this issue.