Preheat: Preheat a grill to medium-high heat (375 to 450 degrees).
Prep the potatoes: Wash the potatoes and pierce them all over with a fork. Place each potato on a sheet of foil and brush it all over with melted butter. Wrap the potatoes in the foil, and pierce the foil several times with a fork.
Grill: Place the foil wrapped potatoes directly on grill. Cook 30 minutes, then flip them. Cook 20 to 30 minutes more depending on the size, until tender when pierced with a paring knife or until the internal temperature is 210 degrees Fahrenheit when measured with an instant read thermometer or grill probe. (As a rule of thumb, 1-pound potatoes should take roughly 1 hour to grill: but it varies based on size, exact grill temperature, and other foods that may be on the grill.) Season with additional butter, salt and pepper, or other toppings as desired.