Background : Composite lymphoma (CL) is a rare disease that has been identified in recent literature. The term composite lymphoma was first proposed to denote the occurrence of more than one lymphoma in a single patient; however, the present accepted definition is the occurrence of 2 or more distinct lymphoma types in a single anatomic site. The condition could be concurrent or sequential. Unlike disease progression or transformation in lymphoma, CL should include two distinct clones proven by morphological and laboratory tests. Pathogenesis : No single definite mechanism has been suggested to explain the pathogenesis of the different types of CL. The etiology is variable, complex and differs according to the types of lymphomas involved. Several theories were proposed including clonal selection with additional mutational accumulation, genomic instability with genetic predisposition, common precursor cell and the aid of a viral factor, mostly EBV. Diagnosis : The morphologic criteria must be confirmed by one or more tests including immunohistochemistry, flow cytometry, gene rearrangement by PCR, cytogenetics, FISH, in-situ hybridization, DNA sequencing and cDNA microarray . Results are more accurate using the laser capture microdissection method. Many combinations of CL are reported, including : Multiple B-cell lymphomas; B-cell and T-cell lymphomas; NHL and HL; or complex B-cell, T-cell and HL cases. Conclusion : Due to the great advancement in molecular characterization of lymphoma, CL is being increasingly identified. It must be carefully diagnosed, because the multiple disease entities may have entirely different natural histories, prognosis and treatment modalities. Also, careful study of such cases may clarify the possible pathogenic mechanisms of the interrelationship of clonal evolution in lymphoma. Key Words : Composite lymphoma -EBV.