Fibroblasts are often used as a feeder layer for progenitor or stem cells in co-culture systems. In the heart fibroblasts are important for cardiac development, homeostasis, and remodelling. They provide cardiomyocytes and progenitor cells not only with nutrition but also secrete extracellular matrix that forms the microenvironment that ensures cell survival and function. Although different kinds of mouse fibroblasts have been used in co-cultures (embryonic, skin and cardiac fibroblasts) adult mouse cardiac fibroblasts (AMCFs) create the closest microenvironment to the adult murine heart for culturing adult mouse cardiac progenitor cells. This protocol describes the isolation of cardiac fibroblasts from adult mouse hearts as well as their maintenance in culture.