Research Findings: This study evaluated the effectiveness of 2 professional development interventions aimed at improving the quality of care provided by caregivers in ordinary infant-toddler child care settings, both center- and home-based. In all, 183 participants in a community college course on infant-toddler theory and practice, an in-service community model of a 48-hr course plus various levels of 1-on-1 coaching (participants were randomly assigned to 0, 5, or 15 hr), or a no-intervention comparison group were compared on their changes across 3 time points in their job-related self-efficacy, their knowledge of best practices, and the quality of their interactions with children as assessed by objective observers. Results indicated that the group with the most intensive intervention (course plus 15 hr of coaching) displayed the most consistent pattern of improvements, which met multiple analytical criteria for substantiveness. Furthermore, these findings were most apparent in the quality of interactions outcomes, and these were further still concentrated in the area of support for language and learning, the domain that showed the greatest need for improvement in infant-toddler caregivers in this sample as well as in previous research. Practice or Policy: Implications of the study results for systemic improvements to infant-toddler care are discussed, such as the importance of individual-level professional development, minimum dosage of coaching, and in-service supports to prevent declines in the quality of teacher-child interactions.