EuroCirCol is a conceptual design study for a post-LHC research infrastructure based on an energy-frontier 100 TeV circular hadron collider. ​The award of the 2013 Nobel Prize for Physics acknowledged the leading role of Europe in particle physics, which has a global community of over 10’000 scientists. To reinforce its pole position throughout the 21st century, Europe must be ready to propose an ambitious post-LHC accelerator project by 2018/19. This is one of the main recommendations of the updated European Strategy for Particle Physics, adopted by the CERN Council in May 2013. The EuroCirCol conceptual design study is a direct response to this recommendation, carried out through a collaboration of institutes and universities worldwide.