Twenty years ago, the European Union launched one of its flagship environmental regulations, the Water Framework Directive (WFD). Since its inception in 2000, the WFD has been a guiding light for water professionals within and beyond the EU; it has pioneered ecological standards for water quality, cycles of river basin management planning, participatory forms of water governance, novel economic instruments, and a recurrent assessment regime. At the same time, the WFD has - by virtue of the far-reaching nature of its interventions aroused political resistance and encountered bureaucratic lethargy; together with many other factors, these have significantly limited its positive impact on the aquatic environment. This Special Issue looks back over the past 20 years to assess what the WFD has achieved, where it has fallen short of expectations, and why. In this introductory piece, the guest editors set the scene and summarise the key findings of the 12 subsequent papers in terms of 6 processes that are characteristic of the WFD's trajectory: implementation, indication, incrementation, inspiration, imitation and insubordination.