Lymphocyte-specific protein tyrosine kinase (LCK) is a member of the Src family of protein tyrosine kinases (PTKs). It contains N-terminal sites for myristylation and palmitylation, a PTK domain, and SH2 and SH3 domains which are involved in mediating protein-protein interactions with phosphotyrosine-containing and proline-rich motifs, respectively. LCK has emerged as one of the key molecules regulating T-cell functions. Studies using LCK knock-out mice or LCK-deficient T-cell lines have shown that LCK regulates the initiation of TCR signaling, T-cell development, and T-cell homeostasis. Because of the crucial role of LCK in T-cell responses, strategies have been employed to redirect LCK activity to improve the efficacy of chimeric antigen receptors (CARs) and to potentiate T-cell responses in cancer immunotherapy. In addition to the well-studied role of LCK in T cells, evidence has been accumulated suggesting that LCK is also expressed in the brain and in tumor cells, where it actively takes part in signaling processes regulating cellular functions like proliferation, survival and memory. Therefore, LCK has emerged as a novel druggable target molecule for the treatment of cancer and neuronal diseases.