This chapter reviews why infants and children are particularly vulnerable to many of the most common environmental hazards. There is some over­ lap with Chapter 2 but the concern here is which environmental hazards pose particular risks for female and male infants and children, and how the relative importance of different hazards changes at different ages and at different points in children's development. Enormous gaps remain in basic data and in our knowledge - for instance about the dose-response relationships for toxic chemicals and about the interaction of environmen­ tal factors with the many social, economic, political and demographic factors which also influence health.1 In addition, data are often not disag­ gregated on the basis of gender, but in the small number of instances where data are available, the significance of gender as a variable in understand­ ing environmental health is apparent.