Beef tenderloin should not be cooked beyond medium (warm pink center) but it is best cooked medium rare. Be vigilant and use an instant read thermometer starting about 15 minutes before the end of the cooking time stated in the recipe. Insert the thermometer in the thickest portion of the beef and when it reaches 125 degrees F (rare), 130 degrees F (medium rare) or 135 degrees F (medium), pull it out of the oven to rest. Keep in mind that the internal temperature will rise a bit as the beef rests so the temperatures stated here are about 5 degrees below the desired end temperature. If you watch the temperature closely, it will be juicy and so tender you can cut it with a butter knife. If you prefer beef cooked beyond medium, I would suggest going with a different cut.