Early Care and Education (ECE) faces two major challenges: poor program quality and fragmentation across different types of programs. In an effort to unify and improve ECE, many states have implemented quality rating and improvement systems (QRIS), a policy strategy that establishes program standards organized into different levels and provides support to help programs attain these increasingly rigorous standards. Despite tremendous investments in QRIS, little is known about the type of quality improvements they evoke and the factors that contribute to changes in quality. This study contributes to the research base on QRIS by exploring the ways that Colorado's QRIS influences dimensions of classroom quality and impacts the emerging ECE system. It mobilizes new institutional theory to analyze QRIS implementation. The results indicate that Colorado's QRIS has had a positive, but nuanced, impact: it had an inconsistent influence on process quality, a more substantial impact on structural quality, and a modest impact on the ECE system. The programs clustered into three categories in regards to their response to the QRIS. The first group had the most positive experiences with the QRIS and included programs with low initial ratings. Teachers at these sites reported that they made substantive process and structural quality changes, supported the QRIS' definition of quality, and engaged in bridging activities to incorporate the QRIS' principles into their practice. The second group was ambivalent about the QRIS. These programs, with high initial ratings, made modest improvements to process quality and more significant structural quality changes. Overall they agreed with the QRIS' philosophy but many of their efforts were symbolic. The third group held negative views. These sites had high initial ratings and participants reported that they made little process quality changes and some structural quality changes: the QRIS' principles often contradicted their approach to ECE and their engagement was primarily symbolic. The analysis also explores ways in which Colorado's QRIS contributes to the formation of the ECE field. The study shows the value of using new institutional theory to analyze ECE and it points to implications for ECE quality improvement, the ECE system, future research, and policy. (author abstract)