The lymphoid system is the pillar of the human immune response. It comprises innate immunity covered by NK cells and adaptive immunity by the B and T-cells. Non-Hodgkin lymphoma (NHL) is a disorder of these immune cells. The understanding of these cells from their formation to maturation is necessary for the diagnosis and management of this disease spectrum. T-cell lymphomas consist of a wide variety of uncommon diseases that can be indolent or aggressive and comprise 12% of all NHL. Since NK-cells are closely related to the T-cells, their malignancies are also included in the classification. WHO released a revised lymphoma classification in 2016, which broadly divided T-cell/NK-cell lymphomas as the precursor and mature T-cell neoplasms. The mature subgroup is further divided into leukemic, nodal, extranodal, and cutaneous. NHL can also be divided into indolent or aggressive diseases. Indolent lymphomas are characterized by a long disease course and are generally resistant to standard chemotherapy, whereas aggressive disease usually has an acute presentation with B symptoms and rapid progression. Cutaneous T-cell lymphoma (CTCL) is considered indolent, whereas peripheral mature lymphomas (PTCL), which include the rest, are considered aggressive. Two-thirds of the CTCLs are comprised of mycosis fungoides (MF) and Sezary syndrome (SS). PTCL-NOS (not otherwise specified) is the most common among the PTCL, followed by anaplastic large cell lymphoma (ALCL) and angioimmunoblastic T-cell lymphoma (AITL).