In a skillet, cook sausage over medium heat until no longer pink; drain.
In a greased 9x13 inch baking dish, layer a third of the marinara sauce, half of the ravioli, half of the sausage, and a third of the cheese. Repeat layers. Top with remaining sauce and cheese.
Cover and bake for 30 minutes. Uncover; bake 15-20 minutes longer or until bubbly.
Notes:
Let the casserole stand for 10 minutes before serving.