The literature on whole school approach (WSA) to education for sustainable development (ESD) has mainly been practice-based, lacking empirical and theoretical grounding. Moreover, the existing literature shows that it is difficult to accomplish WSA to ESD. This chapter addresses these issues by grounding WSA to ESD in applied methods and theories from the field of school improvement research and exemplifying with empirical findings of how to build the capacity to reform a local school in line with WSA to ESD. Four identified themes for developing a local school’s capacity are discussed: improvement history of the school, infrastructure of the school organization, improvement processes, and improvement roles. The chapter takes a novel process-oriented perspective and discusses the school improvement process longitudinally from initiation via implementation to reach institutionalization of WSA to ESD. The main conclusion is that it may take 5–8 years to institutionalize WSA to ESD.