Evaluation researchers are tasked with providing the evidence to guide programme building and to assess its outcomes. As such they labour under the highest expectations - bringing independence and objectivity to policy making. They face huge challenges, given the complexity of modern interventions and the politicised backdrop to all of their investigations. They have responded to with a huge portfolio of research techniques and, through their professional associations, have set up schemes to establish standards for evaluative inquiry and to accredit evaluation practitioners. A big question remains. Has this monumental effort produced a progressive, cumulative and authoritative body of knowledge that we might think of as evaluation science? This is the question addressed by Ray Pawson in this sequel to Realistic Evaluation and Evidence-based Policy. In answer he provides a detailed blueprint for an evaluation science based on realist principles.