Abstract: Peripheral T-cell and natural killer (NK)-cell lymphomas are heterogeneous and uncommon malignancies that are characterized by their aggressive behaviors. The current classification of the World Health Organization defines many entities and groups as predominantly leukemic, cutaneous, extranodal, and nodal diseases according to their clinical presentations. This classification is due to the limited knowledge on the patho-biology, particularly the genetic and molecular alterations, of these diseases. However, recent advances in understanding the subsets of T/NK-cell lineage and their differentiation have provided us novel insights into tumor biology. Moreover, novel genetic aberrations and dysregulated signaling pathways are currently being identified. These aberrations and pathways may serve not only as diagnostic or prognostic markers but also as potential targets for the development of novel therapeutic strategies.