T- and B-cells are generated from hematopoietic stem cells through lymphoid intermediates. The interplay of intrinsic and extrinsic signals determines cell fate at the branch point for T- and B-cell lineages and at the post-commitment stage of lymphogenesis. The mature lymphocytes differentiate into effector/memory cells by antigen recognition, and those differentiation steps are also governed by a series of cell fate choices and survival signals, which allows cells to acquire distinct effector functions. The identification of the molecular details that dictate lymphocyte development and differentiation will improve understanding of acquired immune responses. Recent studies have revealed that Notch molecules, evolutionarily conserved transmembrane receptors, play key roles in lymphocyte development and differentiation. In this article, we review recent knowledge regarding the roles of Notch signaling in controlling both lymphocyte development and acquisition of effector functions.