The goal of this fellowship is to examine the stability of early childhood environmental ecologies and how instability in these ecological settings affect childhood health and well-being and broader patterns of population-level inequality. Prior literature investigating childhood inequality and ecological instability has typically examined point-in-time exposure to ecological contexts or focused on instability in only one domain. This study extends the existing literature by taking a dynamic, multi-dimensional, and child-centered approach to the short- and long-term developmental implications of ecological instability. This study bridges conceptual ideas from sociology, demography, and developmental psychology to accomplish this goal. In this effort, this study asks three main questions: (1) what does instabiliy in the work-family-care nexus look like during the first five years of children's lives; (2) do children influence their own experience of ecological instability during early childhood, and if so what factors (e.g., physical health, social behavior) are most likely to affect these settings; (3)is early childhood instability associated with children's middle and late childhood health and well-being? Moreover, an important component of this research is to examine how these trends differ by children's socioeconomic status, which can speak to how ecological instability shapes the intergenerational transmission of inequality. In answering these questions, this study draws on two complementary sources of data: The NICHD Study of Early Child Care and Youth Development (SECCYD) and the Fragile Families and Child Wellbeing Study (FFCW). These datasets allow for the close examination of changes in family structure, mothers' work characteristics, and children's extra-familial care because of the detailed collection of information during frequent interviews with families over early childhood. In addition, both datasets have objective and subjective measures of children's development, including their physical health, behavioral, and academic outcomes in middle and late childhood. The analytic plan uses latent class analysis (LCA) to identify the ecological instability experience over early childhood, and cross-lagged structural equation models and regression analysis to examine the role children play in influencing their ecological contexts and how early childhood instability may be associated with children's poorer health and well-being in middle and late childhood. Additional statistical techniques will be used to address selection bias and causal inference. This theoretically grounded study has important implications for policy. Understanding what types of instability put specific groups of families at risk for subsequent instability in other domains and which instability profiles are associated with poorer childhood health and well-being can inform policy and interventions targeting vulnerable children and families and speak to larger trends in population health and well-being inequalities.