The Max Planck Institute for Mathematics mourns the death of Michael Atiyah, one of the giants of mathematics. Over many decades Michael Atiyah was a frequent and welcome visitor, inspiring speaker, ingeneous collaborator, and friend to many of the mathematicians in Bonn. Together with Friedrich Hirzebruch, MPIM's founding director and one of Michael's oldest and closest friends, he laid the foundations for topological K-theory, an important tool in algebraic topology. He almost always gave the opening lecture of the annual Mathematische Arbeitstagung in Bonn, from its beginning in 1957.