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 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 book.