It is most often some kind of discomfort which makes baby cry, the most obvious of which is a soiled diaper or gas. Eliminate both of these before seeking any other cure. It took us three long months of crying to find out there was a product called "Gripe Water" in the UK (and in some British stores in the US) a teaspoon of which helps the baby to bring up that last deep annoying bit of wind which no amount of rubbing and patting on the back can get.