When I had my daughter, who was not an easy baby, this book was recommended to me by the mothers on one of my lists. It's brilliant!!! These authors studied primate babies and found that they went through distinct fussy periods during the first year of life that were consistent in terms of timing. They were able to extend this to human babies and I have found it to be incredibly accurate. When Baby Bean suddenly stopped sleeping late last week and by Sunday, I found myself getting very despairing, I decided to get out the book and, lo and behold, human babies go through their first developmental leap, accompanied by significant fussiness, at 5 weeks old and Baby Bean turned five weeks last Thursday. There is a map in the book that outlines the first year, giving timelines for when the baby is most likely to be fussy or not, which can be very useful information to have. Pk was consistent to it within a day or two the first year and so far, it looks like Baby Bean will be too. It's helpful when booking visits, photographers, etc. and also, it can really help you to understand that nothing is wrong when a normally easy baby becomes a grouchy bear for a few days. I wouldn't want to be without this!