Due to an increasing number of children under the age of three attending early childhood education and care (ECEC), there is an increasing need for high-quality care settings. In addition to primary caregivers, ECEC teachers are important socializers of children’s emotional development and may become children’s attachment figures. Early preventive interventions bear the potential to reduce the risk for later maladaptive development in the social-emotional domain. The first objective of the present article is to introduce the Papilio-U3 program, a universal preventive intervention program to foster positive and sensitive teacher-child interactions, to foster children’s early social-emotional competence and secure child-teacher attachment relationships in ECEC, and to prevent children’s emotional and behavioral problems. Furthermore, we report the design and first results of the formative evaluation derived from the pilot evaluation study. The program was developed according to the Intervention Mapping Approach, and the present article describes the planning process comprising a needs assessment and description of the intervention context, the derivation of the logic model of change, a detailed account of the program design, production, implementation, and an overview of the program evaluation study (multi-centric, randomized-controlled trial). The total sample of the evaluation study comprised 125 teachers (age M= 34.0, SD = 10.1, 98.4 % female) from 55 ECEC centers. For the formative evaluation, teachers of the intervention group (n = 61) reported on training content, methods, material, organizational conditions, and potential challenges in the course of program implementation. Finally,we provide directions to revise the program and to guide future implementation of preventive intervention programs in the ECEC context.