Since the 4th Series of this Fascicle was published, there have been considerable advances in our understanding of tumors within the serosal membranes, in particular the classification of mesothelial tumors and their genetics. This 5th Series Fascicle aligns with the terminology recommended by the WHO in its most recent edition for pleural and pericardial tumors, with expansion to reflect serosal membranes at other sites and including recognition of mesothelioma in situ.