Billings sits at the junction of Interstate 90 and Interstate 94, making it the easiest city in Montana to reach by road. I-90 runs east-west and connects to Bozeman, Missoula, and eventually Seattle to the west, and the Black Hills of South Dakota to the east. I-94 heads northeast toward North Dakota. From the south, I-90 comes up out of Wyoming through Sheridan.
Billings Logan International Airport is on the north side of the city and has service from several major carriers including United, Delta, American, and Frontier. It is the busiest airport in Montana.
Greyhound has a stop on the east side of town. If you are thinking about Amtrak, the nearest station is in Malta, about 212 miles north -- not practical unless you are making a trip of it.