Purpose: Preserving and caring for nature in a sustainable way has long been part of Swedish preschool practice. However, recently has it been explicitly referenced in the Swedish preschool curriculum with the goal of incorporating sustainability holistically and coherently in all areas of children's daily preschool experiences through play, care, and education. Research has revealed a potential roadblock to achieving this goal; the difficulty connecting the three inextricably linked dimensions of sustainability: ecological, social-cultural, and economic. The purpose is to generate knowledge about preschool teachers' professional understanding of education for sustainable development (ESD) in early childhood education (ECE) by implementing and enhancing the operationalization of such work in four Swedish preschools. Design/Approach/Methods: The design of the study rests on the involvement of teachers in work where they created sustainability activities supported by the "Environmental Rating Scale for Sustainable Development in Early Childhood" (ERS-SDEC). The data consist of teachers' narratives and interviews, and a qualitative content analysis is used from which a number of themes emerged. Findings: The initial interviews foregrounded three themes: the value of collective resources, the wonders of nature, and a global world. At the end of the project, the teachers' expressions of their practice revealed three qualitatively transformed themes: broadened awareness of sustainability, ecology embracing culture, and expanded perspectives, as factors of ESD. Originality/Value: Teachers did increase their awareness of working with children and expressed a strength in using each other as collective resources. By that, their professional understandings of ESD in preschool developed.