Recent policy efforts in early care and education (ECE) have aimed to ensure that all children have access to high-quality ECE programming. However, there is debate about how to most effectively improve ECE quality at scale. The three papers in this dissertation provide new insights into the impacts of two widely-used policies aimed at improving the quality of children's experiences in ECE: professional development (PD) for in-service teachers, and state ECE accountability systems known as Quality Rating and Improvement Systems (QRIS). In the first paper, I leverage data from a multi-site randomized study of an early childhood PD program to examine whether teachers' classroom quality is affected by their colleagues' participation in PD. I find that teachers with more colleagues assigned to participate in a coaching intervention had higher levels of classroom quality relative to teachers with fewer participating colleagues. Effects were observed regardless of teachers' own participation in the PD, suggesting the presence of both spillover effects and benefits to collective participation. Next, I move to a state-level perspective to examine the impacts of state QRIS on ECE quality. In the second paper, I conduct a difference-in-differences analysis to examine how the expansion of state QRIS has impacted program quality within Head Start. I find little evidence that states' adoption of QRIS led to improvements in program quality in Head Start. States' adoption of QRIS increased annual rates of teacher turnover in Head Start programs but did not improve multiple aspects of structural or process quality. In the third paper, I examine ECE programs' responses to financial incentives for quality improvement in the context of one state's QRIS. I conduct descriptive and quasi-experimental analyses examining programs' responses to both the opportunity to apply for improvement grant funding and programs' receipt of funding. Findings suggest that financial incentives to support quality improvement had only limited impacts on programs' progress through the state's QRIS. Together, findings from these papers provide insight into how ECE policies can improve the quality of children's experiences in ECE, and highlight challenges in improving ECE quality at scale. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]