My daughter turns 11 months old tomorrow and is not yet crawling. I know that every child develops at their own rate, but when do I become more concerned?
When pediatricians assess development, we look at several dimensions at once: cognitive skills, language and social skills, and motor skills. This bigger picture is usually more helpful in deciding whether there is a problem than one skill alone.
Though isolated skills are important, certain ones have much more significance than others. For example, visually following an object in a complete arc by four months is key and more important than the exact month a baby rolls over. In terms of a big picture, crawling isn't a very important one. In fact, some children learn to pull to stand and begin to walk without ever crawling.
More importantly for your child is that her other nine-to-12 month skills are falling into place. She should know her name, she should be babbling with vowels and consonants and speaking one or two words, she should be able to pick up a small object like a Cheerio with her thumb and forefinger neatly, she should look down to see where an object that fell off her high chair (demonstrating that she knows it still exists somewhere, though she can't see it directly), and she should sit well and be able to pull herself up to a standing position. If you feel that some of these other skills are lagging in your daughter, than ask your pediatrician for a more thorough developmental evaluation.