Executive Summary Butterflies have been systematically monitored in Europe for several decades using standard protocols that are now adopted in over 30 countries. Butterflies are ideal biological indicators: they are well-documented, measurable, sensitive to environmental change, occur in a wide range of habitat types, represent many other insects, and are popular with the public because of their beauty. Records from over 6,000 standardised butterfly transects are gathered into a central database as part of the European Butterfly Monitoring Scheme (eBMS) run by Butterfly Conservation Europe (BCE) and the UK Centre for Ecology and Hydrology (UKCEH). The database provides trends on individual species which can be combined to give trends for different habitats. In this report, trends for 17 species have been used to generate an updated Grassland Butterfly Index (GBI) for 1991-2023, using data from all 27 EU countries. The GBI is specifically mentioned in the EU Nature Restoration Regulation (NRR) in article 11 on the Restoration of Agricultural ecosystems as one of the possible indicators that should be measured until the satisfactory levels as set in accordance with Article 14(5) are reached. In practice, for those Member States choosing the GBI as an Indicator, under NNR Article 11, this means that 2025 will be the first year for this indicator. The EU Grassland Butterfly Index shows a decline of just over 50% since 1991. In North-western Europe, the decline is primarily attributed to habitat loss resulting from the intensification of agricultural grasslands and nitrogen deposition in nature reserves. In Northern (Scandinavia, Finland and the Baltic states), Eastern and Southern Europe, the abandonment of grasslands is also a strong driver as shrubs and secondary forest encroachment result in less habitat for grassland butterflies. The decline observed over the last 33 years probably reveals only part of the historical decline in grassland butterflies, as many populations were extirpated from the landscape before 1990 The GBI is the Indicator on the EU Dashboard Target 5, for evaluating progress with improving agroecosystems for biodiversity. It is also part of the EU Sustainable Indicator set, for evaluating the implementation of the Global Biodiversity Strategy. This technical report provides an important message from scientists to policy makers - that butterflies are still declining at an alarming rate across the EU and that urgent action is required to protect and restore habitats to reverse this trend, not only for butterflies but also for other wild insect pollinators and their ecosystem services. The completion and appropriate management of the Natura 2000 network across Europe is a crucial step in helping grassland butterflies. Restoration and creation of landscapes with mosaics of habitats, both within and outside Natura 2000 areas, are essential to protect grassland butterflies in the EU. We are grateful to all the many thousands of volunteer butterfly recorders who contribute their records to this important database, to the many BMS schemes, co-ordinators and funders who support the work, and to the EU for funding the EBMS through the EMBRACE project.