This review of research literature explores the possible harmful effects of day care programs on children's health and physical development, motor development, cognitive development, and social and emotional development, and suggests how variations in staff/child ratios may influence these effects. Two broad categories of conditions are identified as being harmful to children: directed techniques in which the caregiver is being actively harmful and general sensory deprivation which can be active but often takes the form of benign neglect. Findings indicate harm is an elusive concept: children can be harmed physically, but psychological harm requires severe deprivation over extended periods of time before consequences become irreversible. Child/staff ratios are examined in relation to child characteristics, setting (family-style or center-based), program philosophy, caregiver characteristics, and level of standards (for minimal vs. optimal quality programs). Findings indicate that low child/staff ratios tend to produce quieter, less aggressive children who perform better on standardized tests. It is suggested that the Federal Interagency Day Care Requirements (FIDCR) should focus on specifying the actual number of children working with a caregiver, rather than an overall ratio of all children to all caregivers in a program. (CM)