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In this paper we review some of the fundamental properties of the free groupand give a detailed account of Stallings's theory of automata, a geometricinterpretation of its subgroups that has been (and still is) immenselyfruitful, both as a means of understanding classical results, as as a source ofnew results. We review some of the most important ones. -- - En aquest articlerevisem algunes de les propietats fonamentals del group lliure i fem unaexposici\'o detallada de la teoria dels aut\`omats de Stallings, unainterpretaci\'o geom\`etrica dels seus subgrups que ha estat (i segueix essent)immensament fruct\'ifera, tant com a mitj\`a per entendre resultats cl\`assics,com com a font de nous resultats. N'expliquem alguns dels m\'es rellevants.

Aut\`omats de Stallings, un cam\'i d'anada i tornada