Collated indices are a relative measure of butterfly abundance across sites monitored as part of the UK Butterfly Monitoring Scheme. Data from all survey sites (standard UKBMS transects, Wider Countryside Survey transects and targeted species surveys such as timed, larval web and egg counts) are used in the calculation of these indices. The statistics are presented as log10 values. These values are centred round an arbitrary value of 2 as a mean for the time series in order to help show which years are below or above average. Collated indices are calculated annually for each individual butterfly species that has been recorded on five or more sites in that year. Indices are calculated at UK level and at individual country level for England, Scotland, Wales and Northern Ireland where sufficient data are available. Based on this criterion, collated indices have been calculated for the entire time series from 1976 (UK, England and Wales), 1979 (Scotland) and 2004 (Northern Ireland) to the current year for the majority of species, but for some rarer species this has not been possible in some years, particular those in the first part of the time series. Collated indices are calculated using a log-linear model incorporating individual site indices from all monitored sites across the UK or country for a given species in a given year. The number of sites for each species ranges from 5 to several hundred or more and fluctuates from year to year. By 2010 almost 2,000 sites were monitored in total across the UK, with this number rising to more than 3,000 over the next decade. Collated indices are calculated so that we can determine how butterfly populations are changing over time across the UK. This data can be used, for example, to determine where to target conservation efforts and more generally the condition of the UK countryside. Butterflies are recognised as important indicators of biodiversity and environmental change, for example in UK and country Biodiversity Indicators, and have been used in numerous studies of the impacts of climate and habitat change on biodiversity. The UK Butterfly Monitoring Scheme is organized and funded by Butterfly Conservation (BC), the UK Centre for Ecology & Hydrology (UKCEH), the British Trust for Ornithology (BTO), and the Joint Nature Conservation Committee (JNCC). The UKBMS is indebted to all volunteers who contribute data to the scheme. This work was supported by the Natural Environment Research Council award number NE/R016429/1 as part of the UK-SCAPE programme delivering National Capability. Full details about this dataset can be found at https://doi.org/10.5285/7869969c-ac2d-450e-97c6-79d3d6a22a57