Lymphomas are a heterogeneous group of malignancies arising from the clonal proliferation of B-, T-, and natural killer cells at different stages of maturation. Mature lymphomas are classified into Hodgkin lymphoma (HL; 10% of cases), and non-Hodgkin lymphomas (NHL; 90% of cases). NHL are divided into B-cell (85–90%) and T/NK-cell (10–15%) lymphomas, and these are further classified based on maturation stage, phenotypic aspect, histomorphologic features, and genetic findings. For clinical purposes, lymphoma is termed as aggressive (when it grows rapidly and may be fatal within months if not treated, but is responsive to chemotherapy and often curable), and indolent (when it grows slowly over many months to years and is not typically curable with standard approaches, but patients often survive for long even if the disease is not eradicated). A correct diagnosis requires tissue biopsy assessed by a qualified hematopathologist evaluating lymph node/tissue architecture and performing immunophenotyping studies, so that specific subtypes can be identified and prognosis and management can be properly defined. PET/CT in some cases helps to identify the best site for diagnostic biopsy, it is the most accurate instrument to precisely define disease stage in FDG-avid lymphomas, to evaluate the response to therapy, and in some instances to guide continuation of therapy during treatment.