Although the B-cell antigen receptor (BCR) factors most prominently in the maintenance and differentiation of mature B cells, it is now appreciated that co-receptor molecules can positively or negatively modulate signals through the BCR. Co-receptors are functionally defined as modifiers of BCR engagement and signal transuction, and are distinct from other accessory molecules that act independently to regulate B-cell growth. The co-receptor CD19 functions to augment signals by the pre-BCR/BCR and in doing so can modulate B-cell fate decisions at multiple stages of development. In mature B cells, CD 19 also associates with complement receptor 2 (CR2/CD21) and is privotal for transducing signals inducdd by co-recognition of complement C3d-fixed antigens by the BCR and CD21. In this article, we focus on recent progress in the understanding of CD 19 function through the characterization of mouse models that relate in vivo function to biochemical properties of CD 19.