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 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.