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