All butterflies species native to Europe or naturalised before AD 1500 were included in the assessment. Forty-seven species that are of marginal occurrence in Europe were considered in this assessment, but were classed as Not Applicable. An additional species has been introduced in Europe in the late 1980s and is also considered as Not Applicable. Butterfly taxonomy largely follows the 2010-revision of the Taxonomy Commission of Butterfly Conservation Europe, lead by Rudi Verovnik and Martin Wiemers and comprised of Emilio Balletto, John Coutsis, Ole Karsholt, Otakar Kudrna, Miguel López Munguira, Erik J. van Nieukerken and Niklas Wahlberg.
Data were gathered through a questionnaire sent to all national focal points of Butterfly Conservation Europe, asking these specialists to review the species data for their country. These data were compiled to update the Butterfly Conservation Europe database and preliminary assessments were made for each species through a working group of ten experts (Chris van Swaay, Irma Wynhoff, Rudi Verovnik, Martin Wiemers, Miguel López Munguira, Dirk Maes, Martina Šašić, Theo Verstrael, Martin Warren, Josef Settele).
The following data were entered into the database:
A workshop with 50 national and species experts was organised on 28-29 January 2009 in Laufen (Germany) to review the preliminary assessments on a biogeographical basis. Preliminary species summary reports were distributed to all the participants before the review workshop to allow them to check the data presented and prepare any changes to the data. The preliminary assessments were reviewed during the workshop and new information was added to the species summaries and maps. Red List Categories were then defined for each species at the European and EU 27 levels. In August 2009, a meeting was held in Ankara (Turkey) between Resit Akçakaya, Chris van Swaay and several members of Butterfly Conservation Europe in order to discuss how to take into consideration uncertainty in the data analysis and in the resulting Red List categories. Following this meeting, the butterflies assessments were reviewed once again and adjustments were made. Following the review workshop and the uncertainty discussion, the data were edited, and outstanding questions were resolved through communications with the experts. Consistency in the use of IUCN Criteria was checked by IUCN staff from the IUCN Red List Unit. The resulting finalised IUCN Red List assessments are a product of scientific consensus concerning species status and are backed by relevant literature and data sources.