In the insert of a slow cooker, combine the taco seasoning and tomatoes. Add the chicken and toss it in the tomato mixture to coat.
If using a slow cooker, cook for 6 to 8 hours on low or 4 to 5 hours on high. If using an Instant Pot, cook on high pressure for 10 minutes. Let the pressure naturally release for 10 minutes, and then manually release the rest of the pressure.
Shred the meat with two forks, or if you want to be speedy, use a hand mixer set on medium just until the chicken separates into pieces of the desired size.
Let the chicken cool, then pull off the meat and use it in your recipe for tacos, tostadas, quesadillas, nachos, salads, and anything else!
Store in the refrigerator for up to 4 days or store in the freezer for later use.