 from 1982–87 and again served as head from 2011–2018.He has been a member of the Institute for Advanced Study, (IAS) the Mathematical Sciences Research Institute (MSRI), and the American Institute of Mathematics (AIM).He served as the Executive Director of the American Mathematical Society (AMS), the Chair-elect, Chair, and Retiring Chair of the Mathematics Section of the American Association for the Advancement of Science, as a member of the Advisory Board of the American Institute of Mathematics, as a member of the Board of Mathematical Sciences at the National Research Council/National Academy of Sciences, as a member of the Joint Policy Board for Mathematics.