The Water Framework Directive (WFD)(2000/60/EC) aimed to improve water quality throughout the European Union. In 2000, the text was fixed, but the text’s meaning remained fluid. The goal of the WFD may be the same for all European water bodies – reaching ‘good status’ or ‘good potential’ – each country decides, within limits, how to define “good” and how to reach this goal. Actual implementation of the WFD emerges from actions by changing associations of actors, which keep transforming the WFD meaning through their agency. This paper discusses one of the projects in the Netherlands that has been mobilized within WFD implementation. As of 2005, a modelling tool to assess effectiveness of measures for water bodies – the WFD Explorer – was developed to make (scientific) expertise accessible for policy developers and decision makers. The Explorer was to have a role in harmonising practices for WFD implementation and in coordination processes. As it stands now, however, the Explorer is not accepted by several prospective users – which re-introduces diversity as other tools with similar objectives are being developed. The WFD asked for qualifications of water bodies in terms of the substance water (including chemical and physical properties), the infrastructure of water, and flora and fauna in that water. The (perceived) properties of the materiality of the substance were negotiated within the WFD and the Explorer. Explorer development saw huge debate about which (sets of) rules needed to be mobilized in order to compute effects of measures on water quality. The Explorer required scientific knowledge to be consolidated before incorporating it in an instrument, which proved to be extremely complicated. Although Latour may not have had such an instrument in mind when writing Science in Action (1987), developing a policy-support instrument while the knowledge required is still very much contested is exceedingly value-laden.@en