Pleural and peritoneal mesotheliomas (PPM) are rare cancers mostly related to asbestos-exposure, which are characterized by a complex histopathological diagnosis and staging, few therapeutic options and a dismal prognosis. The main unmet medical need in PPM is the lack of a treatment sequence for affected patients. The advent of immune checkpoint inhibitors has changed the first line treatment of PPM, thanks to the improvement in survival achieved by the combination of ipilimumab and nivolumab that are currently approved for non-epithelioid histology in our Country. PPM is characterized by a large heterogeneity of the genomic landscape, which is mainly characterized by the loss of tumour suppressor genes and mutations in DNA repair genes and by an altered- suppressed or excluded tumor immune microenvironment (TIME). The goal of this project is to improve the immune-biological and molecular stratification of PPM subgroups that can lead to the identification of different personalized treatment approaches. PPM patients (N=220) will be retrospectively (N=150) and prospectively (N=70) recruited from the coordinator center and 6 participating Italian centers. Treatment-naïve tumor samples will be collected and analyzed by bulk gene expression and spatial whole transcriptome analysis, and by 9-color multiplex immunofluorescence. New targets or actionable pathways potentially emerging from such studies will be finally assessed and validated in patient-derived organoids/xenografts that accurately reflect PPM tumorigenesis.