The consensus is "back to sleep"for as long as possible to prevent SIDS. However, some babies sleep better on their tummies and some doctors say that it is OK as long as the babies have good head/neck control. DD learned to sleep on her tummy when she learned to rolled from back to front and vise versa(4-5 mos). I wasn't concern as she can always rolled back to her back. I always put her on her back to sleep and she would roll/crawl around to a comfortable position before falling asleep. HTH