There has been an increasing focus on children as a special population in the fields of toxicology and epidemiology. At the same time, there has been considerable improvement in the technology for defining normal development and pathways of pathogenesis. Increased support of these areas has culminated in stronger research programs and greater professional involvement in addressing the specific challenges of applying new techniques and data to the improvement of children's health. Part of these challenges relates to the ever changing environment of the child. Not only does a child's anatomy, physiology, and metabolism change with time, but their lifestyle and awareness change as well. All of these can have a significant impact on a child's exposure and the potential of that exposure to have an effect on health and development. This paper will provide a brief overview of the susceptibility of the child relative to sensitive developmental life stages, the changing nature of exposure parameters during development, and how these factors can impact the relevance of predictive biomarkers of chemical toxicity in children.