PrefaceThere are many reasons to welcome this collection of papers and to applaud the support given by the Directorate of the Economic and Social Research Council (ESRC) Teaching and Learning Research Programme (TLRP) for the work that has led to its production. The TLRP has been concerned with the question of how to demonstrate the 'warrant' of its wide range of research projects, not only to satisfy the complex but intrinsic obligations of educational research as a multidisciplinary field but also to fulfil the expectations of decision-makers in education-from classroom teachers to ministers of state-for assurance and practical guidance about what sorts of knowledge and evidence they can have legitimate confidence in.These are wholly defensible, indeed desirable, expectations on the part of those charged with either teaching or policy-making, given the amount of so-called research that is accessible and downloadable from the Internet. As Conor Galvin's work has made plain (Galvin, 2003(Galvin, , 2004(Galvin, , 2005) the opportunities (temptations, even) for policy and practice to be based on info-nuggets and evidence-lite, even downright misinformation, are growing all the time. I have argued elsewhere (for example, Saunders, 2004Saunders, , 2007 that, however hedged about with theoretical and practical reservations and qualifications it may be, the rationalist ideal in public decision-making is to be cherished. The alternative is that public life be dependent on revealed or idiosyncratic knowledge, in which case there would be no bulwark even in principle against opportunism, corruption, ignorance, solipsism and collective amnesia. So the question of 'warrant', the assurance that the grounds of our belief are solid under our feet, is crucial to the continued and strengthened influence of educational research on education in an increasingly competitive knowledge market-place.To judge by the approaches that have been taken by different TLRP project teams to warranting their findings, this assurance may be thought to rest on, for example, the protocols of peer review, on methodological propriety, on analytical cogency or on the utility and plausibility of the research findings and messages for non-research audiences. The papers in this volume attempt to place these different approaches in a more general and theoretical context and, in so doing, to fulfil two rather different needs. The first is to set out-for non-specialists-the underlying reasons why different kinds of research in education can and should be deployed as forms of trustworthy knowledge, together with the conditions under which they may be trusted. This has entailed saying some very interesting and useful things along the way about the contexts, criteria, underpinning values, constraints and limitations, and the making of meanings, in educational research. The second need is to explicate some specialist topics and issues about the enduring concerns of 'validity',