External expertise plays an important role in European Union (EU) policymaking. In an increasingly complex multi-tiered environment, EU policymakers are requested to provide timely responses on a variety of issues in which several interests are at stake. External expertise provides the EU institutions with a way to address these issues efficiently and build political consensus.Attempts to regulate the role of external expertise in EU policymaking date back as far as the early 2000s, when the European Commission released the first guidelines for the use of expertise and advice in policymaking. In 2005 a register of expert groups was established. By June 2016, a total of 27 199 members were recorded on that register. These members made up 820 'Commission expert groups'.The role of experts and external advisers in EU policymaking remains controversial. Concerns include the neutrality, transparency and balance of external expertise. Following a 2014 European Ombudsman investigation, the European Commission announced that a revised register of expert groups would be operational by the first quarter of 2016. Revised rules for the expert groups were adopted at the end of May 2016. Further changes have been introduced by the Interinstitutional Agreement on Better Law-Making between the European Parliament, Council and Commission. The own-initiative report being drafted by the Parliament's Committee on Budgetary Control may also influence the way the system evolves.