The internal temperature of roast beef should be 145°F (63°C) for medium-rare, 160°F (71°C) for medium, or 170°F (77°C) for well done. It is important to use a meat thermometer to check the internal temperature of the roast beef to ensure it is cooked to the desired level of doneness. When checking the temperature, insert the thermometer into the thickest part of the beef until it reaches the desired temperature. It is also recommended to remove the roast beef from the oven when it reaches 5 to 10 degrees lower than the desired temperature, as it will continue to cook and rise in temperature while resting. After removing the roast beef from the oven, tent it with foil and let it rest for 15-20 minutes before carving to allow the juices to redistribute and the meat to finish cooking to the desired degree of doneness.