When liquids are cooled below their melting temperature T-m, sometimes they do not solidify immediately but remain in supercooled state up to temperatures far below the melting point. Under certain conditions, they can solidify into the form of amorphous solids without crystallizing. A supercooled liquid and a amorphous solid are metastable phases, which are not completely understood in terms of structural arrangement and thermodynamic behaviour. But by far the most interesting feature is the glass transition which is the manifestation of a true thermodynamic transition and a dynamic event since a dramatic dynamic arrest intervenes. A unified theory of supercooled liquids and glass transition does not yet exist and, more specifically, the link between the sharp increase of the relaxation time and the correlation length is a question still largely open. This article presents in the most elementary manner a brief overview of this delicate issue.