One consequence of two ideas in British history, evidence and liberty, is experimental criminology. Experimental criminology applies research designs developed in Britain to matters of liberty affecting the entire world. The promise of experimental criminology is to generate better evidence about how to increase liberty. With an increasing number of criminologists willing to undertake experiments, the promise can continue with even greater impact of evidence in promoting liberty. There are two possible models discussed for applying experimental research. First, bottom-up discretion is a model in which crime victims, police, judges, and probation officers and others take experimental results into account when making their decisions one case at a time. Second, top-down guidance is a model in which either Ministers or an independent evidence-assessment agency, like the National Institute of Health and Clinical Excellence, appoint committees of practitioners and researchers to develop succinct operation guidance based upon systematic reviews of experimental evidence. Bottom-up discretion is seen as growing steadily due to the persistence of criminologists and practitioners doing experiments, and liberty is seen as having the potential to advance much faster by top-down guidance. This article attempts to articulate the promise of experimental criminology. Figures, notes and references