"The Happiest Baby on the Block" is a parenting book written by pediatrician Harvey Karp. It offers techniques to soothe fussy babies and promote better sleep. Karp introduces the concept of the "fourth trimester," suggesting that newborns need extra comfort and security to transition from the womb to the outside world. The book outlines five calming techniques known as the "5 S's": swaddling, side/stomach position, shushing, swinging, and sucking. These methods aim to mimic the conditions of the womb, helping parents manage crying and create a more peaceful environment for their infants.