Protein kinases are critically involved in signaling pathways that regulate cell growth, differentiation, activation, and survival. Initially identified as a T-cell specific member of the Src family of protein tyrosine kinases, Lck has become the object of intensive investigations which have revealed a key role for this kinase in the central processes controlling T-cell development, activation, proliferation, and survival. Lck is expressed specifically in lymphoid cells. It contains one protein kinase domain, one SH2 domain, and one SH3 domain. It is associated with a variety of cell surface receptors and is critical for signal transduction from the T-cell antigen receptor (TCR). Consequently, Lck is targeted by regulatory proteins of T-lymphotropic viruses, especially by the Herpesvirus saimiri (HVS) tyrosine kinase interacting protein (Tip). This oncoprotein physically interacts with Lck in HVS transformed T cells and has an impact on its catalytic activity. Together with the identification of defects in the regulation of Lck expression or activity in T-cell leukemias, suggests that dysregulation of Lck might play a role in neoplastic transformation. However, under certain conditions, Lck is also involved in the induction of apoptosis. This chemosensitizing effect of Lck is independent of T-cell receptor signaling and does not require the kinase activity of Lck. The findings demonstrate that Lck might be part of two independent signaling pathways leading to either cell proliferation or apoptosis.