  Some model-independent properties of the effective string of gauge field
systems in the confining phase , for very large quark separations, are
described in terms of two-dimensional conformal field theories. The constraints
induced by the gauge theory at the boundaries of the effective string induce a
Coulomb- like term in the interquark potential which is universal, but
different from the one proposed by L\"uscher. Some universal relations among
the string tension, the thickness of the colour flux tube, the location of the
deconfining temperature and a lower bound of the glueball spectrum are
discussed. (Lectures given at the XXXII Cracow School of Theoretical Physics,
Zakopane, Poland 2-12 June 1992)
