Crawling is your baby's first method of getting around efficiently on his own. In the traditional crawl, he'll start by learning to balance on his hands and knees. Then he'll figure out how to move forward and backward by pushing off with his knees. At the same time, he'll be strengthening the muscles that will soon enable him to walk.
Most babies learn to crawl between the ages of 7and 10 months. Your baby may opt for another method of locomotion around this time, though – like bottom shuffling (scooting around on her bottom, using a hand behind and a foot in front to propel herself), slithering on her stomach, or rolling across the room.
Don't worry about her style – it's getting mobile that's important, no matter how your baby does it. Some babies don't crawl and move directly to pulling up, standing, cruising (furniture walking), and walking.
Since the "Back to Sleep" campaign was initiated in 1994, many babies seem to be crawling later and a few skip it completely. (The campaign aims to reduce the risk of SIDS by encouraging parents to put babies to sleep on their back.) If your baby's using her arms and legs equally on both sides of her body and learning how to coordinate them, you probably have nothing to worry about.
Your baby will likely start crawling soon after he's able to sit well without support (probably by the time he's 8 months old). After this point, he can hold his head up to look around, and his arm, leg, and back muscles are strong enough to keep him from falling on the floor when he gets up on his hands and knees.
Over a couple of months, your baby will gradually learn to move confidently from a sitting position to being on all fours, and he'll soon realize he can rock back and forth when his limbs are straight and his trunk is parallel to the floor.
Somewhere around 9 or 10 months, he'll figure out that pushing off with his knees gives him just the boost he needs to go mobile. As he gains proficiency, he'll learn to go from a crawling position back into a sitting position.
He'll also master the advanced technique that pediatrician William Sears calls "cross-crawling" – moving one arm and the opposite leg together when he moves forward, rather than using an arm and a leg from the same side. After that, practice makes perfect. Look for him to be a really competent crawler by the time he's a year old.