Mesothelium consists of a monolayer of specialized cells known as mesothelial cells that form during embryonic development over internal organs and line peritoneal, pleural and pericardial cavities. Mesothelial cells have been shown to undergo epithelial-mesenchymal transition (EMT) and during embryonic development differentiate into a variety of cells including vascular smooth muscle cells in the heart, lungs and intestine, fibroblast cells and cardiomyocytes. Recent studies have shown that the mesothelium of the heart, the epicardium, contributes to the coronary vasculature during embryonic development and is involved in repair mechanisms in injury models of the heart and liver. In order to determine which tissues and organs mesothelial cells contribute to during embryonic development and how they maintain adult tissue homeostasis, we have established a lineage tracing strategy using a mesothelial-specific Cre (Recombinase) mouse line. In order to demonstrate a stem/progenitor cell role of mesothelial cells, Tamoxifen was administrated to pregnant or adult mice carrying the mesothelial-specific CreERT2 construct, in combination with a Rosa26-based reporter, for short and long term pulse-chase experiments. Our results indicate that mesothelial-derived cells contribute to a range of tissues in the developing embryo and in the adult. Specifically, in embryos, mesothelial cells contributed to the formation of vascular structures in developing kidneys. Furthermore, in adult mice, our analysis revealed that Tamoxifen pulse induced recombination is dose and route of administration dependent, leading to significant recombination of reporter alleles which can be detected weeks after Tamoxifen injection. As a consequence of mesothelial-specific lineage tracing, we found clonal distribution of labelled cells in the mesothelium of all internal organs, including the heart, lungs, intestine, spleen, and kidney. Here, we will present our current understanding of the dynamics of mesothelial cell contribution to tissue maintenance and regeneration. Further, we will discuss that duration and route of Tamoxifen administration are important parameters that need to be considered during Tamoxifen-induced lineage tracing studies.