Peripheral T-cell lymphoma (PTCL) is a heterogeneous group of diseases characterized by an aggressive clinical course and resistance to standard chemotherapy regimens. PTCL comprises approximately 10-15% of all lymphomas in the Western world, although it could be found as high as 30-40% in Asian countries. Several clinical, pathological, genetic, and molecular prognostic factors have been described in order to risk stratify and treat our patients more appropriately. However, due to the rarity of these conditions, most studies are small in size and retrospective in nature. The present chapter goes in depth into the multiplicity of prognostic factors described in patients with PTCL, according to the most common subtypes, such as PTCL, not otherwise specified, anaplastic large cell lymphoma, angioimmuno- blastic lymphoma, and adult T-cell f/lymphoma, among others. Our hope is that larger, prospective studies will be done to clarify the role of prognostication and modeling of therapy for patients with these hard-to-treat lymphomas.