Microbiologists Brett Finlay and Marie-Claire Arrieta explain how the trillions of microbes that live naturally in and on our bodies influence childhood development; why an imbalance of those microbes can lead to obesity, diabetes, and asthma, among other chronic conditions; and what parents can do-- from conception on-- to positively affect their own behaviors and those of their children. They describe how natural childbirth, breastfeeding, and solid foods influence children's microbiota. They also offer practical advice on matters such as whether to sterilize food implements for babies, the use of antibiotics, the safety of vaccines, and why having pets is a good idea.