Preheat grill to medium heat. Individually wrap the unpeeled plantains in aluminum foil and place on the grill.
Let them cook for about 20 to 25 minutes, until they are soft and cooked through. You may also cook them on the upper rack of the grill at a different temperature, but it may take more or less time.
You know they are ready when they feel extremely soft to the touch and the sugar has begin to caramelize.