• Bonn-Cologne Algebra Seminar, Germany, June 2009.
• Emmy Noether Center, University of Erlangen-Nürnberg, Germany, Apr. 2009.
• St. Nicholas Conference, RWTH Aachen, Germany, Dec. 2008.
• Colloquium, Mathematics Department, RWTH Aachen, Germany, Nov. 2008.
• CombinaTexas, El Paso, Texas, Apr. 2008.
• Special Session on "Arrangements and Related Topics", Co-organizer, AMS/MAA Joint Meetings, New Orleans, Jan. 2007.
• Special Session on "Invariant Theory", AMS/MAA Joint Meetings, New Orleans, Jan. 2007.
•  MSRI (Berkeley), "Recent Developments in Arrangements and Configuration Spaces" honoring Sergey Yuzvinsky's 70th birthday, August, 2006.
• Texas Tech Univ., Outstanding Early Career Speaker, "Invariant Theory in Perspective", Red Raider Symposium, Nov. 2004.
• MSRI (Berkeley), "Combinatorial Aspects of Hyperplane Arrangements",  Nov. 2004.
• Texas Christian University, Colloquium, Oct 2004.
• Texas A&M, Combinatorics Seminar, April 2004.
• U.C. Santa Barbara, Automorphic Forms Workshop, March 2004.
• Microsoft Corporation and Univ. of Washington joint seminar, March 2004.
• Phoenix: AMS-MAA National, Special Session, Jan. 2004.
• U. of Goettingen, Germany, "INGO2003: Invariant Theory and its Interactions with Related Fields", March 2003.
• Louisiana State University, AMS Sectional, Special Session, March 2003.
• U. of Wisconsin, Madison, AMS Sectional, Special Session, Oct. 2002.
• University of Oregon, Colloquium, June 2002.
• University of Santa Clara, California, Colloquium, Feb. 2002.
• U. of Heidelberg, Germany, Algebra Seminar, June 2001.
• U. of Minnesota, Combinatorics Seminar, May 2001.
• U.C. Berkeley, Combinatorics Seminar,  April 2001.
• Columbia U., AMS Sectional, Special Session "Arrangements of Hyperplanes", Nov. 2000.
• U. of Wisconsin, Madison, Lie Theory Seminar, Nov. 2000.
• U. of Wisconsin, Madison, Topology Seminar, Nov. 2000.
• U.C. Davis, California, Bay Area Discrete Math Day, Oct. 2000.
• U.C. Santa Cruz, California, Colloquium, Oct. 2000 and Jan. 2000.
• Valparaiso U., Indiana, Colloquium, Jan. 2000.
• Tokyo Metropolitan U., Japan, Colloquium, Nov. 1999.
• San Antonio, AMS-MAA National, Special Session "Geometry in Dynamics", Jan. 1999.