BACKGROUND: There is a growing body of evidence for serious health consequences of exposure to ambient air pollution. The general question of who is susceptible is one of the most important gaps in current knowledge regarding particulate matter (PM)-related health effects. Who is susceptible depends on the specific health endpoint being evaluated and the level and length of exposure. Here, we restrict the review on the impact of fine particle exposure on children's health to the following outcomes: infant death, lung function, respiratory symptoms and reproductive outcomes. METHODS: This is a strategic review of children's susceptibility to ambient fine particles and characteristics of infant and children which underlie their increased susceptibility to PM. RESULTS: Ambient fine PM is associated with intra-uterine growth retardation, infant mortality; it is associated with impaired lung function and increased respiratory symptoms, particularly in asthmatics. Concerning infant mortality, exposure to PM is strongly and consistently associated with postneonatal respiratory mortality and less consistently with sudden infant death syndrome. Although most of the studies reported adverse effects for this health outcome, the evidence is weaker than for infant death. Exposure to fine PM has been associated with impaired lung function and lung function growth. Most of the studies reported increased prevalence of symptom with increased exposure to fine PM. CONCLUSION: Fine PM is a major threat to children, because of their higher exposure to PM compared to adults, the immature state of the lung in childhood and also of the immune function at birth. The first months of life might be a period of particular sensitivity. Although the mechanisms of air pollution effects have not yet been completely understood, pregnant women, infants and children need specific protection against exposure to fine particles.