For decades, it has been known that the serous cavities, which includes the peritoneal, pleural and pericardial cavities, harbour large numbers of macrophages. In particular, due to the ease of isolating these cells, the peritoneal cavity has been used as a convenient source of macrophages to examine many facets of macrophage biology over the last 50-60 years. Despite this, it is only recently that the true heterogeneity of serous cavity mononuclear phagocyte compartment, which includes macrophages and dendritic cells, has been revealed. Advances in technologies such as multi-parameter flow cytometry and the ‘OMICs’ revolution have uncovered the presence of distinct populations of mononuclear phagocytes in the serous cavities. Given that peritoneal macrophages have been implicated in many pathologies, including peritonitis, pancreatitis, endometriosis and acute liver injury, it is imperative to understand the biology of these cells. Here, we review the recent advances in understanding the identity, origin and function of discrete serous cavity mononuclear phagocyte subsets in homeostasis and how these may change when homeostasis is perturbed, focusing on peritoneal and pleural cavities and highlighting differences in the mononuclear phagocytes found in each.