This report details the findings from the first phase of a multiyear study examining the prevalence, nature, and predictors of placement instability in substitute care. By combining administrative data analyses of the placement histories of over 200,000 children in care in Illinois with findings from a web-based survey of 1,192 child welfare caseworkers, this study represents one of the most comprehensive efforts to date to examine the issue of placement instability. The findings reveal that placement movement was due to a mix of factors and circumstances. Workers reported that over three-quarters (75.9%) of children's most recent placement moves were due, at least in part, to foster parents' inability or unwillingness to continue fostering. Among those moves attributed to foster parents, the reason most commonly cited was foster parent's inability to tolerate children's behavioral or emotional problems (27.6%). The study also found that, while placement in relative foster homes and placement with siblings significantly reduced the likelihood of subsequent placement instability, a large percentage of prior moves had been attributed to efforts to move children to these types of placements. Taken together, these findings suggest that the average number of placements children experience while in care could be effectively reduced by placing them with relatives at entry to care, which would afford children the stability of relative homes without requiring them to endure a subsequent change in placement.