This chapter presents a state of the art summary of mature T and NK neoplasms to its biology, epidemiology, disease mechanisms, clinical signs, diagnosis, treatment and prognosis. The most common mature T and NK neoplasms are cutaneous T-cell lymphomas, extranodal natural killer/T-cell lymphoma nasal type, angioimmunoblastic T-cell lymphoma, adult T-cell leukemia/lymphoma, enteropathy-associated intestinal T-cell lymphoma, anaplastic large cell lymphoma, and peripheral T-cell lymphoma not otherwise specified. Patients with mature T and NK neoplasms often show painless swelling of lymph nodes, persistent chills/fever, night sweats, unexplained weight loss, lack of energy, and itching. Treatment of mature T and NK neoplasms involves chemotherapy, radiation therapy, biologic therapy, bone marrow or stem cell transplantation, and surgery. Diagnosis of mature T and NK neoplasms relies on clinical, laboratory, radiologic, and pathologic examinations. Extranodal NK-/T-cell lymphoma, nasal type is thought to originate from mature NK cells as the vast majority of tumor cells express an NK cell phenotype.