You have two priorities when reheating tamales: preserve the quality of the delicate masa and bring the filling it holds up to 165 degrees Fahrenheit, the minimum safe temperature for leftovers. Whenever you reheat delicate food, you should use a gentle reheating method; with tamales, steaming works best. Careless reheating results in a crumbled, watery mess rather than a fresh, hot tamale on your plate. It's best to use an instant-read thermometer to check the temperature of the filling, but as long as it's hot and steaming, it's safe to eat.
Fill a pot with a couple inches of water and set the steamer insert in it. Bring the water to a boil.
Stand the tamales in the steamer rack, open end up. Cover the pot and reduce the heat to a simmer.
Steam refrigerated tamales for 10 to 15 minutes or until they reach an internal temperature of 165 degrees F. Steam frozen tamales 15 to 20 minutes, or until they reach an internal temperature of 165 F.