2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/34611Scalar-tensor theories are the best motivated alternatives to general relativity and provide a mathematically consistent framework to test the various observable predictions. They can involve three functions of the scalar field: (i) a potential (as in "quintessence" models), (ii) a matter-scalar coupling function (as in "extended quintessence", where it may also be rewritten as a nonminimal coupling of the scalar field to the scalar curvature), and (iii) a coupling function of the scalar field to the Gauss-Bonnet topological invariant. We recall the main experimental constraints on this class of theories, and underline that solar-system, binary-pulsar, and cosmological observations give qualitatively different tests. We finally show that the combination of these data is necessary to constrain the existence of a scalar-Gauss-Bonnet coupling.8 pages, 2 figures, uses "moriond.sty", contribution to the XXXVIIIth Rencontres de Moriond on Gravitational Waves and Experimental Gravity, Les Arcs (France), March 23-29 2003General Relativity and Quantum CosmologyAstrophysicstext