This article reports the development and evidence for validity and application of the Adjustment Scales for Early Transition in Schooling (ASETS). Based on primary analyses of data from the Head Start Impact Study, a nationally representative sample (N = 3077) of randomly selected children from low-income households is configured to inform developmental-transitional stability and change in socioemotional adjustment. Longitudinal exploratory and confirmatory factor analysis of the ASETS revealed behavioral dimensions of Aggression, Attention Seeking, Reticence/Withdrawal, Low Energy, and higher-order dimensions of Overactivity and Underactivity. Each dimension was vertically equated through IRT, with Bayesian scoring across 2 years of prekindergarten, kindergarten, and 1st grade. Multilevel modeling provides evidence for concurrent validity, assessment of future risk, and detection of differential growth trajectories across the 4 years of early school transition. (Contains 7 tables and 2 figures.)