This study seeks to answer the following question: Are impacts on Head Start classroom quality associated with impacts of Head Start on children's learning and development? This study employs a variety of descriptive and quasi-experimental methods to explore the role of classroom quality as a mediator or mechanism of Head Start impacts. This research uses data from the Head Start Impact Study (HSIS) and includes 4,440 3- and 4-year-old children who were randomly assigned off a waitlist to either receive an invitation to participate in Head Start services or to the control group. Children initially applied to 351 Head Start programs across 81 Head Start grantees. A total of 2,644 children were randomized to receive Head Start services and 1,796 were randomized to the control group. Following randomization, children enrolled in a total of 1,632 classrooms across 930 Head Start and non-Head Start centers. Measures of children's cognitive skills include early receptive language, math skills, and early literacy. Classroom quality was assessed using three tools: The Early Childhood Environment Rating Scale (ECERS-R), The Family Day Care Rating Scale (FDCRS), and the Arnett Caregiver Interaction Scale (CIS). Results of the multinomial logit models indicate that random assignment to Head Start is indeed associated with increases in all three classroom quality factors. Future analyses will address critical questions about the extent to which these impacts on quality are associated with impacts on child outcomes, using a variety of analytic approaches. Tables and figures are appended.