Roast beef should be cooked to an internal temperature of 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 temperature of the beef, and to remove the beef from the heat 5-10 degrees before reaching the desired temperature, as it will continue to cook during resting time. The resting time should be around 15-20 minutes, which allows the juices to redistribute and the meat to finish cooking to the desired degree of doneness.