Context .- Malignant mesothelioma is a relatively rare pleural tumor that may mimic benign mesothelial lesions and various other tumors including carcinomas and sarcomas. This makes the diagnosis challenging for the pathologist. Objective .- To provide a brief but useful update on the immunohistochemical, cytogenetic, and molecular markers that are currently available for the diagnosis of malignant mesothelioma. Data Sources .- Reference materials including peer-reviewed publications, text books, and consensus opinion reports among pathologists. Conclusions .- It is important to correlate histologic findings on adequate biopsy samples with clinical and radiologic features. Useful diagnostic mesothelial markers include calretinin, WT-1, cytokeratin 5/6, and D2-40 (podoplanin). It is recommended that at least 2 mesothelial and 2 carcinoma markers with greater than 80% sensitivity and specificity be used for the diagnosis of mesothelioma when all clinical, radiologic, and histologic features are concordant. p16 deletion is reported in up to 70% of primary epithelioid and 90% to 100% of sarcomatoid pleural mesotheliomas. Presence of this homozygous gene deletion is so far the best indicator of mesothelioma. To date, this deletion has not been reported in any benign mesothelial lesion. The impact of various histologic patterns on the clinical and prognostic aspects of mesothelioma is addressed. The pleomorphic pattern, when present in more than 10% of tumor, translates into a highly aggressive behavior and is associated with poor survival. Recent studies have shown that the high-grade subgroup of deciduoid mesothelioma with pleomorphic histologic pattern also has a more aggressive clinical course. Nuclear grade (combination of nuclear atypia and mitotic count) may also prove to be an independent prognostic factor.