In this chapter, I contrast two school cultures—assessment culture (AC) and testing culture (TC)—and examine their potential to support or inhibit assessment for learning. First, I compare the two cultures with regard to the mindset that each culture reflects and then with regard to classroom learning, teacher professional learning, and leadership, addressing their interrelationships and their effects on formative school-based assessment. I further compare the impact of external accountability tests on AC and TC schools. In the discussion, I contrast the schools with regard to an underlying dimension of internal coherence, addressing the conditions that foster and those that hinder such coherence. I then explain why AC, as opposed to TC, is conducive to the successful implementation of assessment for learning, and identify the challenges in transforming a TC into an AC. I conclude with recommendations to guide such a transformation.