Slice peppers and onions then place them on the bottom of your slow cooker. Place chicken on top of peppers and onions. Sprinkle taco seasoning over the chicken and pour the diced tomatoes over the top.
Cover and cook on LOW for 5-6 hours OR on HIGH for 3-4 hours. Remove chicken and cut into strips or shred with 2 forks; return to the Crockpot.
Serve with tortillas and your choice of toppings: Lettuce, tomato, cheese, salsa, sour cream, guacamole, etc.