The function of the surface antigen receptor of B cells (BCR) has been extensively studied with respect to the activation of mature B lymphocytes. B cells at other points of development (e.g., pre-B cells, immature B cells, and germinal center B cells) also express forms of the BCR, and for cells at these developmental timepoints, signaling through the BCR in some cases may lead to outcomes other than B-cell activation. Understanding the molecular events that are initiated by BCR crosslinking would enhance our understanding of the regulation and functional results of BCR signaling throughout B-cell development. In this article we review the current understanding of BCR signal transduction from initiation of the signal through changes in expression of genes that regulate the activation state. Costimulatory and modulatory molecules are considered with regard to their ability to affect the sensitivity or outcome of the BCR signal. Finally, we discuss how BCR signal transduction and the results of BCR signaling may differ at distinct stages in B-cell development.