The diagnosis of diffuse pleural mesothelioma requires in most cases a pleural biopsy, performed either under imaging guidance (ultrasound or computed tomography) or thoracoscopy. Loss of BAP1 or MTAP expression (immunohistochemistry) and homozygous deletion of CDKN2A (fluorescence in situ hybridization) are the basic molecular markers for the diagnosis of mesothelioma. The histologic type and patient's performance status are the most important prognostic factors. Pleural effusion can be managed by the insertion of tunneled pleural catheters, either as a stand-alone measure (e.g., patients not amenable to multimodality therapy who have been diagnosed by pleural fluid cytology or image-guided biopsy) or combined with the administration of aerosolized talc during a diagnostic thoracoscopy. Immunotherapy is one of the front-line approaches in inoperable patients, particularly in biphasic or sarcomatous histologic varieties.