Every baby is unique and all babies develop at different rates. Typically, a baby will be rolling from front to back by about six or seven months.
Back to tummy is the next stage, which gradually leads to crawling. Your baby's first attempts to crawl may involve pulling their legs up beneath them while lying on their tummy, and pushing up with their hands. But that's all they will manage - a crawling position, without the crawl! (Some babies get very frustrated at this stage.)
By about nine months your baby spends most of their time rocking backwards and forwards in a crawling position, raring to go but still not moving. They may squirm or roll across the floor, but have no control over where they end up.
Between nine and 10 months most babies finally start to move, but sometimes backwards because their arms are stronger than their legs.
They often end up further away from the interesting object they've seen, rather than closer.
Most babies learn to crawl on their hands and knees, but some prefer a bear-like walk on their hands and feet - which looks funny but is often just as fast. A few lie flat on their tummies and pull themselves along with their arms 'commando-style'. Others learn to bottom-shuffle, going straight to pulling themselves up, missing out conventional crawling altogether. Each method is fine, although 'bottom-shufflers' and 'commandos' often walk later.
When your baby is up to speed, they can reach two kilometres an hour, and may cover about 200 metres a day. This means you'll now need to have 'eyes in the back of your head' to prevent accidents. See Safety issues.
"Now that she's learnt to crawl, Eleanor (at 10 months) likes to get hold of the newspaper - the sports section - lie on her back and rip it to shreds. She gets covered in newsprint. The other day I could read her left cheek!"