All cells continually survey their environment and make decisions based on cues encountered. This requires specific receptors that detect such cues, then transduce signals that initiate the appropriate responses. B lymphocytes provide an archetypal model for such 'adaptive' cellular responses, where signals transmitted by the B cell Ag-receptor (BCR) influence not only cellular selection, maturation, and survival, but are imperative in generating the ultimate effector function of B cells, i.e. antibody production. While other extracellular stimuli and their cognate receptor signals can also influence B cell development, BCR-mediated signals and the way in which they are integrated and regulated are paramount in defining the cell's physiological fate.