This is considered a ‘cry’ method of sleep training. This technique entails allowing your baby to cry while checking on him at intervals. The goal here is to reassure him every so often that you are nearby and to reassure yourself that he is okay. When you go to check on your baby, you are not “supposed” to pick him up nor engage him much, but simply reassure him using your voice and a loving pat for 2-3 minutes, tops (watch the clock!). With this sleep training method, the goal is NOT to help baby fall asleep – that is what he is learning to do on his own! Instead, the idea is that he falls asleep on his own, in the same “environment” in which he will awaken periodically throughout the night. The knowledge of how to fall asleep unassisted at bedtime will pave the way for him/her to go BACK to sleep throughout the night. Over time, you gradually increase the amount of time between your ‘checks’. See a more detailed step-by-step explanation of this method here: The Ferber Method
"My first daughter was sleeping through the night (10 p.m. to 9 a.m.) by 6 months. We had a complete bedtime routine: a bath, a book, a bottle, then to bed, a little music in the crib, and asleep in 10 minutes. It was wonderful, but that scenario didn't work for my second daughter and hasn't worked for my son, so I've tried different things for each of them. Sometimes a plan doesn't work. Listen to your baby – he or she will tell you what you need to know."
Crying isn't the goal of this sleep training method, but advocates say it's often an inevitable side effect as your baby adjusts to sleeping on his own. They say the short-term pain of a few tears is far outweighed by the long-term advantages: A child who goes to sleep easily and happily on his own, and parents who can count on a good night's rest.
If your baby is older than six months, don’t worry, McGinn says: “It’s never too late to develop good sleep habits.” Dickinson says he finds nine months to be a bit of a sweet spot for parents in terms of getting babies to sleep through the night. “They are at a good age for understanding routines and don’t need to eat during the night,” he explains.