2007: Member of the Institute for Advanced Study, Princeton
(Additive Combinatorics special semester).
June 2001: BBV Professor at Universidad Autonoma, Madrid, Spain.
1999: Kloosterman Professor at University of Leiden, Holland.
Fall 1994: Visitor at The University of Michigan in Ann Arbor.
April 1993: Visitor at Universidad Autonoma, Madrid, Spain.
1993: Member of the Isaac Newton Institute, Cambridge, England.
( L-Functions and Arithmetic special half-year).
June 1989: Visitor at J.E. Purkyne University, Brno, Czechoslovakia.