Cardiac fibrosis is characterized by the accumulation of extracellular matrix proteins in the heart, contributing to both systolic and diastolic dysfunction. This review discusses the cellular and molecular mechanisms involved in cardiac fibrosis. Activated myofibroblasts are the main effector cells, but other cells like monocytes/macrophages, lymphocytes, mast cells, vascular cells, and cardiomyocytes also contribute by secreting fibrogenic mediators. Inflammatory cytokines, reactive oxygen species, mast cell-derived proteases, endothelin-1, the renin/angiotensin/aldosterone system, matricellular proteins, and growth factors like TGF-β and PDGF are key mediators. Experimental and clinical evidence suggests that cardiac fibrotic changes may be reversible. Understanding the mechanisms of cardiac fibrosis is crucial for developing anti-fibrotic treatments. The normal cardiac interstitium consists of three constituents: the epimysium, perimysium, and endomysium. Cardiac fibroblasts are the most abundant interstitial cells, playing a key role in maintaining the matrix network. Myofibroblasts, derived from fibroblasts, are crucial in the fibrotic response. The origin of myofibroblasts in cardiac fibrosis is debated, with possible sources including resident cardiac fibroblasts, circulating fibroblast progenitors, epicardial epithelial cells, and endothelial cells. Myofibroblasts are activated by various signals, including TGF-β, and contribute to fibrosis. Monocytes and macrophages play a role in the fibrotic response, with different subpopulations having pro-fibrotic or anti-fibrotic effects. Mast cells release fibrogenic mediators and contribute to fibrosis. Lymphocytes, particularly Th2 and Th17 cells, are involved in fibrosis. Endothelial cells may promote fibrosis through pro-fibrotic mediators or by undergoing endothelial to mesenchymal transition. Cardiomyocytes can contribute to fibrosis through cell death and signaling. The extracellular matrix in the fibrotic heart includes fibrillar collagens, non-fibrillar collagens, fibrin, fibronectin, and matricellular proteins like TSP-1, TSP-2, TSP-4, tenascin-C, SPARC, and OPN. These proteins modulate fibrotic responses and matrix remodeling. Understanding these mechanisms is essential for developing therapies to treat heart disease.Cardiac fibrosis is characterized by the accumulation of extracellular matrix proteins in the heart, contributing to both systolic and diastolic dysfunction. This review discusses the cellular and molecular mechanisms involved in cardiac fibrosis. Activated myofibroblasts are the main effector cells, but other cells like monocytes/macrophages, lymphocytes, mast cells, vascular cells, and cardiomyocytes also contribute by secreting fibrogenic mediators. Inflammatory cytokines, reactive oxygen species, mast cell-derived proteases, endothelin-1, the renin/angiotensin/aldosterone system, matricellular proteins, and growth factors like TGF-β and PDGF are key mediators. Experimental and clinical evidence suggests that cardiac fibrotic changes may be reversible. Understanding the mechanisms of cardiac fibrosis is crucial for developing anti-fibrotic treatments. The normal cardiac interstitium consists of three constituents: the epimysium, perimysium, and endomysium. Cardiac fibroblasts are the most abundant interstitial cells, playing a key role in maintaining the matrix network. Myofibroblasts, derived from fibroblasts, are crucial in the fibrotic response. The origin of myofibroblasts in cardiac fibrosis is debated, with possible sources including resident cardiac fibroblasts, circulating fibroblast progenitors, epicardial epithelial cells, and endothelial cells. Myofibroblasts are activated by various signals, including TGF-β, and contribute to fibrosis. Monocytes and macrophages play a role in the fibrotic response, with different subpopulations having pro-fibrotic or anti-fibrotic effects. Mast cells release fibrogenic mediators and contribute to fibrosis. Lymphocytes, particularly Th2 and Th17 cells, are involved in fibrosis. Endothelial cells may promote fibrosis through pro-fibrotic mediators or by undergoing endothelial to mesenchymal transition. Cardiomyocytes can contribute to fibrosis through cell death and signaling. The extracellular matrix in the fibrotic heart includes fibrillar collagens, non-fibrillar collagens, fibrin, fibronectin, and matricellular proteins like TSP-1, TSP-2, TSP-4, tenascin-C, SPARC, and OPN. These proteins modulate fibrotic responses and matrix remodeling. Understanding these mechanisms is essential for developing therapies to treat heart disease.