3 to 5 months
At about 4 months, your baby figures out that with the help of his stronger torso muscles, he can turn from his stomach to his side. Not long after that, he manages (often accidentally) to roll from his side to his back. Although some infants learn to roll in the opposite direction first -- from back to belly -- for most that's a later accomplishment. In a few more weeks, your baby will likely be able to make it all the way around, flopping from belly to back to belly again. Then he may roll across the room as a way to travel.