2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/106255These are lecture notes of a C.I.M.E. course I gave at Cetraro, June 6-11 2005. The theory described is the version of Chen-Ruan's Gromov-Witten theory of orbifolds developed by Graber, Vistoli and me in the algebraic setting, but with introduction beginning in Kontsevich's formula on rational plane curves and through Gromov-Witten theory of algebraic manifolds. As this is not a joint paper, I finally get to give direct credit to Vistoli's beautiful ideas! In the appendix, I include, with permissions, two relevant letters Kontsevich wrote to Borisov in July 1996. People who study motivic integration might want to read and understand something they have found confounding.A confusing typographical error fixed (thanks to Eric Katz)Algebraic Geometry14N35; 14A20; 53D45text