Recent research has drawn attention to the deleterious effects of instability on child development. In particular, child care instability may make it hard for children to form secure attachments to their care providers which may have a negative impact on their development and school readiness. These effects seem to be heightened for low-income children and families. However, there remains a lack of clarity regarding how and why low-income mothers make changes to their child care arrangements. Using ethnographic data from Welfare, Children, and Families: A Three City Study, this study explored 36 low-income mothers' experiences of child care instability and stability and the factors that promoted each. We identified four kinds of child care transitions: planned, averted, failed, and forced. Financial resources, transportation and the availability of care during the hours that mothers work were important for helping mothers find and maintain preferred care arrangements. Our findings have implications for research on child care instability as well as the development of policy and programs to help low-income families secure high quality child care and maintain stable employment. (author abstract)