Yellow marrow is a type of bone marrow found in the long bones of the body, primarily composed of fat cells. It serves as an energy reserve and plays a role in the production of certain hormones. Unlike red marrow, which is responsible for producing blood cells, yellow marrow is less active in hematopoiesis, especially in adults. In children, most bone marrow is red, but as they grow, some of it converts to yellow marrow. This transformation helps the body store fat and maintain energy levels. Yellow marrow can also be found in the femur and humerus, among other bones.