Japanese knotweed, Fallopia japonica var. japonica (Polygonaceae) is one of the most pernicious invasive weeds in Europe. A vigorous, herbaceous perennial, it was introduced from Japan to the nursery of Philipp von Siebold in the Netherlands in 1849 and was subsequently made widely available to the European public, soon becoming a much-prized ornamental. In the UK, its history has been well ... [Show full abstract] documented and shows an invasion pattern which is typical of many Victorian introductions; becoming naturalised by the late 1880s, it was first recorded in the wild in Maesteg, South Wales, in 1886. It has subsequently increased its range to include most regions of the UK, with a further northward spread anticipated due to climate change. Its status as a weed was soon recognised, and today it is one of only two terrestrial plants which are “illegal to cause to grow in the wild” under the UK 1981 Wildlife and Countryside Act, as well as being classed as a 'controlled waste', meaning that a licence is required for its disposal. The success of Japanese knotweed as a weed is all the more impressive when one considers that its expansion, at least in GB has been achieved without the advantage of sexual reproduction, which normally contributes to long-distance dispersal. It is believed that only a male-sterile clone of F. japonica was introduced into Europe and its spread is the result of disturbance, such as flood events, which move rhizome fragments along river systems, as well as human activity, in particular the illegal dumping of garden waste and the use of top soil contaminated with rhizome fragments. The plant's extensive and resilient rhizome system makes control very challenging and official knotweed management guidance has been issued in the UK to provide best practice advice to landowners and contractors. Nonetheless, the long term efficacy of traditional control attempts is often found wanting. The costs of Japanese knotweed can be considered as both economic and environmental. Its reputation as a “concrete cracking superweed” is justified and it causes many costly problems in the built environment both structurally and aesthetically. Though harder to quantify, the impacts the weed has on ecosystem function and biodiversity are also considerable, shading out native vegetation and prohibiting - regeneration, reducing invertebrate species richness and consequently impacting the higher food chain. Dense stands can also exacerbate flooding, damage riverbank protection works and impede flow, whilst dead stems can cause blockages downstream when swept away. Knotweed's influence on riparian systems is particularly pertinent in the light of the EU Water Framework Directive, which demands that member nation's waterways achieve “good ecological status” by 2015. With national control costs estimated to be in excess of £1.5 billion if such a task were to be attempted, and the limited arsenal of chemicals permitted for use in its preferred riparian habitats, the only sustainable and viable long-term control option is classical biological control. This article describes the search for a Japanese knotweed specific biological control agent (a psyllid - Aphalara itadori), its evaluation and the request for its use in the UK from 2010.