1. Non-commutative geometry, Nashville, 9-16 May 2005.

2. International congress "Geometric linearization of graphs and groups", EPFL, 22-26 January 2007 (organized with G. Arzhantseva).

3. Affine Isometric Actions of Discrete Groups, Ascona (Switzerland), June 28 - July 3, 2009 (organized with G. Arzhantseva).

4.   Joint meeting of the Korean and American Math Societies, Seoul, 15-23 december 2009.

5. Chaire Francqui, KU Leuven, Mars 2011 (leçon inaugurale le 16 février 2011)