Large granular lymphocytic (LGL) leukemia is a rare lymphoproliferative chronic disorder characterized by expansion of either T- or NK- cytotoxic cells. Contrary to EBV-induced aggressive NK-LGL leukemia, chronic T- and NK-LGL leukemia are indolent diseases affecting elderly patients with a median age of 66.5 years old. LGL leukemia is frequently associated with autoimmune disorders, most frequently rheumatoid arthritis. An auto/allo antigen is tentatively implicated in disease initiation. LGLs expansion is then triggered by proinflammatory cytokines such as interleukin (IL) IL-15, MIP-1, and RANTES. This proinflammatory environment contributes to deregulation of proliferative and apoptotic pathways. Following the initial description of the JAK-STAT pathway signaling activation in the majority of patients, recurrent STAT3 gain of function mutations have been reported. The JAK-STAT pathway plays a key role in LGL pathogenesis by promoting survival, proliferation and cytotoxicity. Several recent advances have been made towards understanding the molecular landscapes of T and NK LGL leukemia, identifying multiple recurrent mutations affecting the epigenome, such as TET2 or KMT2D, and crosstalk with the immune microenvironment, such as CCL22. Despite an indolent course, published series suggest that the majority of patients will eventually need treatment. However, it is noteworthy that many patients may have a long-term observation period without ever requiring therapy. Treatments rely upon immunosuppressive drugs, namely cyclophosphamide, methotrexate and cyclosporine. Recent advances have led to the development of targeted approaches, including JAK-STAT inhibitors, cytokine targeting and hypomethylating agents, opening new developments in a still-incurable disease.