This article develops the analogy of the "black box" in relation to different levels of a professional development project to understand the nature of learning that results in higher student achievement. Principles of formative assessment are used to frame the on-going, evidence-informed inquiry into learning that operated at each level of the system involved: policy, project delivery, including expert facilitators, and school. Data collected throughout the two-year project cycle (of which there were three), included student achievement in reading or writing; student interviews; classroom observations and responses from teachers to scenarios; interviews, and taped examples of facilitator practice. There were considerable effect size gains in student achievement in each of the three, two-year cohorts of schools. To understand these gains the project and its various contexts for learning are examined. Examples of how inquiry into learning at one level impacted on practice at others are discussed.