Abstract : A limit group is a nitely generated group which is model of the universal theory of a free group. These groups were introduced by Z. Sela in his study of the elementary theory of free groups and the solution of the famous problem of Tarski. They coincide with other groups previously studied which are known under the name of fully residually-free groups and which were independently studied by Baumslag, Miasnikov and Kharlampovich. They constitute the rst step in understanding the elementary theory of free groups. One of the important notions used in the study of the elementary theory of free groups is the notion of JSJ-decomposition. The objective of these minicourses is to give an introduction to limit groups and JSJ-decompositions.