Sitting up and crawling begin after that - usually between 6 and 8 months, then walking is soon to follow, but kids walk on their own time table. It can happen anywhere from 10 to 18 months, and that's normal. Factors that can delay these physical milestones include prematurity at birth, having siblings to help baby instead of baby learning how to perform skills himself, natural strengths and temperament. Many kids wait to attempt new skills until they know they can master it; others are daredevils who will try anything once. If you're ever worried about your child's development, ask your pediatrician; don't consult books or the Internet - they'll just drive you nuts with what-ifs, as will comparing your baby to others his age or older siblings when they were babies.