This study revisits the cellular composition of the adult mouse and human heart, revealing that endothelial cells are the most abundant non-cardiomyocyte cell type, comprising over 60% of non-myocytes, while fibroblasts constitute less than 20%. Using genetic tools and cellular markers, the researchers examined the relative frequency of cardiac endothelial cells, hematopoietic-derived cells, and fibroblasts in the mouse and human heart. Immunohistochemistry and flow cytometry analyses confirmed that endothelial cells are the most abundant cell population, with fibroblast numbers being smaller than previously estimated. The study also describes an alternative fibroblast surface marker that more accurately identifies the resident cardiac fibroblast population. These findings challenge the previous assumption that fibroblasts are the most abundant non-cardiomyocyte cell type in the heart. The study provides a comprehensive survey of major cell types in the human and mouse heart, demonstrating that endothelial cells are the most abundant cell type in both species. The results have implications for understanding cardiac development, homeostasis, aging, and injury responses. The study also highlights the importance of accurate characterization of cell types in the heart for future research in cardiac biology.This study revisits the cellular composition of the adult mouse and human heart, revealing that endothelial cells are the most abundant non-cardiomyocyte cell type, comprising over 60% of non-myocytes, while fibroblasts constitute less than 20%. Using genetic tools and cellular markers, the researchers examined the relative frequency of cardiac endothelial cells, hematopoietic-derived cells, and fibroblasts in the mouse and human heart. Immunohistochemistry and flow cytometry analyses confirmed that endothelial cells are the most abundant cell population, with fibroblast numbers being smaller than previously estimated. The study also describes an alternative fibroblast surface marker that more accurately identifies the resident cardiac fibroblast population. These findings challenge the previous assumption that fibroblasts are the most abundant non-cardiomyocyte cell type in the heart. The study provides a comprehensive survey of major cell types in the human and mouse heart, demonstrating that endothelial cells are the most abundant cell type in both species. The results have implications for understanding cardiac development, homeostasis, aging, and injury responses. The study also highlights the importance of accurate characterization of cell types in the heart for future research in cardiac biology.