Preheat oven to 375 degrees F (190 degrees C)
Place chicken breasts in a lightly greased 9x13 inch baking dish. Sprinkle taco seasoning on both sides of chicken breasts, and pour salsa over all.
Bake at 375 degrees F (190 degrees C) for 25-35 minutes, or until chicken is tender and juicy and its juices run clear.
Sprinkle chicken evenly with cheese, and continue baking for additional 3-5 minutes or until cheese is melted and bubbly. Top with sour cream if desired, and serve.