ABSTRACT In many countries, we can see a movement to more large-scale and system-wide approaches to school improvement. The aim of this article is to study and theorize on conditions for school actors’ knowledge building within such large-scale school improvement reforms. Using a Swedish large-scale school improvement program, “Collaboration for the Best School Possible” (CBSP), as a case and the concepts of organizational routines and theorizing as theoretical lenses, we explore questions linked to school improvement design and conditions for school actors’ knowledge building. Internal policy documents from the Swedish National Agency of Education and interviews with responsible officials constitute the empirical material for the study. The results point to a great variation concerning the degree of standardization and formalization of the CBSP design. This implies that some parts of the CBSP process are to a large degree conditional on individuals’ competences, experiences, and attitudes. An important conclusion from the study is that an efficient large-scale school improvement design should contain organizational routines that can reinforce the interplay between the different levels of the school systems, but also create conditions for integrating research to nurture different types of theorizing. Such design will generate conditions for professional knowledge building and educational change.