Mesothelioma is a rare aggressive cancer that develops from the mesothelium. Recent molecular analyses have defined four different types of mesothelioma based on gene expression and two major molecularly-defined groups based on prognosis. In this volume potential mechanisms causing this heterogeneity are explored. The different chapters include heterogeneity learned from experimental animal models in NF2/Hippo pathway signaling stem cell signaling pathways the tumor microenvironment and micro RN