While Education for Sustainable Development (ESD) is increasingly being implemented in educational systems, monitoring projects which capture the status and diffusion processes of ESD are also gaining relevance. The article presents part of the national monitoring of ESD in Germany—a qualitative expert interview study—which aims to analyze the diffusion process of ESD in different educational areas (early childhood education, school education, vocational education and training, higher education, non-formal learning organizations, and local authorities). Its goal is to gain a systematic understanding of the diffusion process of ESD in the different areas of the German educational system. For the analysis of the 66 expert interviews, a qualitative content analysis was used. The overarching result of the study is that there is no single transformation path of ESD diffusion that fits all educational areas. Instead, characteristics of ESD as well as prevailing structural conditions, systemic goals, and the understanding of education within the respective educational area have an impact on the diffusion of ESD. The diffusion process of ESD evolves within this circular interplay of innovation and innovation system. A deeper understanding of it therefore has great potential for practitioners’ (self-) reflections and for further research projects.